I very rarely use them, but if I get frustrated enough or forgot to save before an important battle, I will use it. I will likely use it if I spot a legit shiny and it has a chance of blowing up or I don't have the right Pokémon to whittle its HP down. I used one a few days ago to catch Cresselia before restarting Platinum.

I'll use a Master Ball against roaming legendaries, like Suicune/Raikou/Entei or Latias/Latios for example. I would also maybe use it to capture a shiny if I encountered one, but both times I ran into a legit shiny were too early in the game and I hadn't yet acquired the Master Ball.

I can count the amount of Master Balls I've used on my 3rd hand. 0. I've never used one, and almost never will use one. I'm using one in XD, but it's only for a friend. If it was my game, I'd catch that in any other ball. I'm a collector of them, so any chance I get at one I'll take, but I still like to have fun in catching hard things like legendaries. That's why I'm an expert catcher today, practice, trial, and error. I've even developed a specific strategy for running legendary Pokémon.

I try to use Master Balls as sparingly as possible. I tend to use them a lot when I soft reset to get the right nature on a certain legendary pokemon. Other than that, I don't waste my Master Ball on the main legendary pokemon of the game I'm playing (Such as Reshiram, Palkia, Kyogre, etc.)
However, I usually cave in and use my Master Ball on Shadow Lugia in Gale of Darkness, mostly because my team can't take the shadow attacks that XD001 dishes out. If I tried using different kinds of Poke Balls on XD001, my team would probably get wiped out before I could catch it.

I use masterballs if i have too. Normally i try to catch them the old fashion way, if that dont happen masterball comes in. Back when the first games were out i always used the masterball on mewtwo.

When I was younger, I used my Master Ball on roaming legendaries because I never figured out the proper way to catch them. (either by using a trapping move or ability or whittling them down bit-by-bit) Then I figured those out, and my Present Self was all at Past Self.

Now I generally either don't use it at all, use it on a difficult legendary (Rayquaza, this means you), or waste it on something like a Rattata.

I remember once when I was a kid, I took my brother's Red Version and caught a Dugtrio with his Master Ball because I thought Dugtrio was the hardest Pokémon to catch. Little did I know....

Anyway, here's what usually happens with me and my Master Ball:
"Okay, I only have one of these, so I should save it for something very important *stockpiles Ultra Balls and uses only those on legendary Pokémon*"
It's the same way with rare or one-of-a-kind items in other games. I save them and tell myself to use them for something big, but never end up using them.

I haven't used my Masterball in Platinum, HeartGold and White. I prefer not to use it, since I enjoy the classic way of capturing legends.

I'll only use them on Pokémon I for surely want and when I know I may kill it or it may run. Possibly like shinys if I ever see any. Maybe for a legendary, but that's a rare case now... Sometimes I stockpile it for just one time I may use in the future...

When I was a kid playing Crystal, I wanted to see what the animation was. I used the Master Ball on a Rhyhorn and kept it because of my ignorance and lack of knowledge. Nowadays, I wait to find a shiny or use it on a runaway Legendary.

I usually only use them for roaming legendaries, right now in my copy of Black I received an extra Masterball but because I filled up my National Dex I have no use for it.

I freaked out the first time I caught Rayquaza in a Master Ball because I thought they worked like all other pokeballs at the time.

Before, I use a Master Ball to catch the first legend in the game. In later plays, I attempt to save before battling a legend and use the slow way, and use Master Ball on roaming.

In my SS game, I did not successfully catch Lugia in the old-fashioned way (out of patience), so I concluded the probability formula was changed. I used the Master Ball on the roaming.

Now that I'm doing this super annoying Rayquaza thing, I think I might use my HG Master Ball as cautionary defense measures. It knows Rest AND Outrage, so it could kill itself, or stall out the battle with a thousand rests. I don't have time for that.
Oh yeah, and Shiny Legendaries I'd use a Master Ball on without thinking. Special and I don't have 'em? Waaaant.

Rarely use them. The only time I do use them is for roaming Legendary Pokémon. If I don't have the right Pokémon on my team to prevent it from escaping while lowering it's HP enough, I just say "Okay, that's it bro, I'm lazy", and toss the Master Ball like a frag grenade at the sucker. If there are trios on the run-about, I just pick the one I like best using my Master Ball, and harass the other two later on from time to time.

I use it on whatever Legendary I think is going to be the hardest to catch. In Emerald I used it on Rayquaza. Whatever makes my life easiest!

Ya i recall just using them on the roaming legends latly. Like thundorus/tornadus, Mesprit, The 3 birds, ect.

Master Balls are the kind of items you hoard because they're one-of-a-kind, even though doing so is pointless since it's meant to be used.

Since

there are no roaming legendaries in Black 2/White 2

Hey, once you've developed that certain strategy for roamers it becomes as easy as catching a Magikarp. I think my collection of Master Balls has increased to 10 or 11. Maybe 12. I do restart a whoooole lot.

I've been trying to complete my Pokédex in White, and I'd got everything I needed from Hoenn except a Relicanth. After an hour of searching I finally encountered one, and since my patience was wearing away by that point I just said "**** it" and threw a Masterball. I'm never going to use that file again, anyway, since it's been pretty much emptied aside from some spare Pokémon I had lying around.

I only throw Master Balls at roaming Pokemon, only if I have no access to a trapping move. Although, I got lucky with Tornadus and caught it without using the Master Ball, although I'd usually use it on Latios in Ruby, Entei in Leaf Green, and Mesprit in Diamond. Otherwise, I'd just not use it.

only twice I used it in pearl on palkia cause I had no other balls forgot to buy some and in heart gold same reason on ho-oh cause i had no others never bought them otherwise no I don't need them

Across all of the games, what Pokémon did you use your Master Ball on, and why?

Gen I
Anything and everything after discovering the MissingNo. glitch

Gen II
One of the beasts when I was nearly finished with the Pokédex

Gen III
I want to say Latios in Ruby, but I can't remember.

Gen IV
I used one on Cresselia in Platinum because I wanted to restart and didn't want to chase it around, which got sent forward to White.
I used one on Sudowoodo in SoulSilver because I burned it and ran out of Great Balls. (Someone traded it to me, anyway.)
I used a final one on Entei when I got sick after Raikou took forever.

Gen V
I haven't used any Master Balls yet.

Gen I
Moltres (first play through without knowledge of MissingNo.)

Gen II
Ho-oh

Gen III
I think Rayquaza

Gen IV
Cresselia (Platinum)
Entei (Heartgold)

Gen V
If I recall correctly, I didn't use it on anything.

I used one on Dialga. It was my time playing Pokemon so I gave it some damage because I wasn't convinced that it does 100% catch rate.

Gen I
Mewtwo in R/B usually. I tended to have to use it against a legendary bird in Yellow.

Gen II
Any of the roaming legendary dogs I might run into in G/S/C.

Gen III
Kyogre in Sapphire, Latias (or Latios? I forget) in Emerald. (I had about 50 ultra balls and couldn't ever catch Kyogre...)

Gen IV
Cresselia.

Gen V
I haven't ever used my Masterball in B/W, but I did have to use it to catch Kyurem in Black 2.

Gen I- Nothing
Gen II- Nothing
Gen III- Shadow Lugia in XD (Not a main game, but it's something.)
Gen IV- Nothing (I killed the legendaries before I thought about Master Balls.)
Gen V- Nothing

As you can see, I don't use Master Balls But I sure do enjoy collecting them. I had to use the Master on Shadow Lugia because it would be annoying to catch it regularly and then check its stats and with ALL that text and animation... Master Ball made the pain go away.

Well, if you mean first play throughs...

Gen I: Dugtrio because I thought it was super awesome and hard to catch (when I was 5)
Gen II: Nothing because I still have yet to beat a Gen II game (be right back bashing my head in)
Gen III: Never got the Master Ball because I didn't find it (be right back again)
Gen IV: Diamond - Dialga because the strategy guide I had recommended I use the Master Ball on it. Platinum - I want to say I saved it, but I might've used it on Giratina
Gen V: Never used it (to my knowledge)

Gen 1: Mewtwo
Gen 2: Poliwhirl (I Am DEAD Serious...I was 7 and I pressed A w/o thinking), and Entei a friend traded me another Master Ball
Gen 3: Never Played a Gen 3 Game
Gen 4: Dialga
Gen 5: Thunderus (Didn't want to chase it around)

Gen I
Mewtwo. I completely missed the Birds the first time around- the nest time I got them frozen and caught them frozen. Works. ^^

Gen II
Raikou in Gold, Suicune in Crystal (Though that's not the smart thing to do >.>)

Gen III
The respective Eon Pokemon in R/S, but I didn't use it in Emerald.

Gen IV
I used one on Mespirit in Diamond, and one on Cresseilia in Platnium.
Soulsilver/Heartgold, I used it on one of the legendary dogs- I think it was Entei in both cases.

Gen V
B/W, never used it. B/W2, still never used it.

The way I used mine were weird. Also note that Gen 3 was practically the first Pokémon generation I completed.

Gen I: Articuno
Gen II: Entei
Gen III: Relicanth (Don't laugh, I thought it was rare! >_<)
Gen IV: Cresselia and Giratina in Platinum
Gen V: Kyurem in Black and I almost used one when I encountered a Shiny Azelf in Black 2.

Gen I: Don't own any Gen I games.
Gen II: Don't own any Gen II games.
Gen III: Sapphire- Latias. Emerald- Latios
Gen IV: Diamond- Cresselia. Platinum-Mesprit. Heartgold- Ho-oh. (I had weak Pokemon at the time!)
Gen V: Black- Tornadus. Black 2- Nothing

I only own Sapphire, so I've only used the master ball once. I used it on Kyogre, and that was before I knew Rayquaza was in the game. I figured that Kyogre was in Sapphire, Groudon was in ruby, but they wouldn't be in the opposite games with each other, so I figured that Rayquaza wouldn't be an exception and would only be in Emerald.

Gen I: Zapdos because I ran out of balls ( I was young so I didn't realize to save before batting)
Gen II: Heartgold: I used it on ho-oh because I forgot to by some :/
Soul silver: Raikou because it was so difficult so I got tired of it.
Gen III: Didn't use it
Gen IV: Didn't use it
Gen V: Didn't use it.

Gen I: Missingno (Mewtwo when I didnt know Missingno existed and Mew. Im such a glicth lover) (Yellow)

Gen II: I got lucky and never used it (Crystal)

Gen III: Latios (Emerald)

Gen IV: I dont remember

Gen V: Never (B/W 1, never played 2)

I used my Masterball on Mewtwo.

I COULD have used a different one, but it just felt right to use the Masterball on him.

I reserve it for shinies that have a chance of killing themselves or something of the ilk.

I decided to use mine on Zygrade because I needed the experience but I wish I used it on Mewtwo instead.

I generally don't use the Master Ball because it's one of those "save it indefinitely for things that probably won't ever pop up in the wold like a shiny Trapinch that could probably be caught in a regular Poké Ball anyway" things.
In other words all it does is take up bytes in my inventory.

I used it on my Mewtwo . I attempted catching it with an Ultra Ball a few times and then thought it would be kind of cool to have the M on the Master Ball be M for Mewtwo .

Well, there are two stories that go along with this topic.

When I first got Pokemon, I didn't really know what a MasterBall was, let alone that they were so hard to get! So, I saw a wild Caterpie... and caught it with a MasterBall. Once I found out what I did, I hated my decision! So, I had to catch Kyogre with an Ultraball -_-

Nowadays, I use my Masterball usually for either a roaming legendary or a harder-to-catch legendary. I definitely don't throw it at a Caterpie or Bidoof, but I have used it on a shiny Pokemon! Masterballs in my opinion are overrated.

I view MasterBalls are being created for the story-purpose of catching the cover legendary, so out of respect for that I normally use mine on the cover legendary. Sometimes I'll save it for Mewtwo, and when I've gotten to use the Faraway Island event back in Emerald I save my MasterBall for Mew, because the animation is pink and it's a handy M for Mew!

I have a time limit: If I go over an hour on capturing a Pokemon (or I run out of balls lol), I use it. It's only happened with Moltres on SoulSilver.