What are your opinions on the GTS? A useful, a half-half or a useless addition?

Here's my personal opinion:

I find the GTS, as a whole, a half-half addition; it allows you to obtain rare Pokémon, events and legendaries, thus making it easier to complete the Pokédex. However, it is full of hacks, doesn't display important information about the Pokémon  [like the nature, moves, ability, etc.], and doesn't offer good options when depositing a Pokémon (like the nature/ability wanted, etc.)

The fact is that things were so much harder without it. It could use a lot of improvements, but its still a great idea.

I have noticed some problems in Platinum which I hope were fixed in HGSS. For example, it no longer shows the level requirement. Some Pokémon (I heard about this a while ago, but only noticed it once for the first time about a week ago) also are glitched up, specifically they are named something but are actually something else. I'm not sure if this is a problem with the actual Pokémon, only with how its being displayed to the person searching for Pokémon, or if someone is trying to clone it actively.

There was something that has been called a glitch, but whether it is or not, it wasn't fixed in Platinum (but was probably discovered in the first day of DP's release), so might be intentional - when you deposit a Pokémon into the GTS, make a trade with someone else there, and withdraw it, it will evolve if it is capable of doing so through trade (I have done this many times - it helps a lot).

There are two very, very simple things I would like to see - the ability to view the full summary of the Pokémon you are trading for, and strong "unfairness" checking - making sure the Pokémon is not hacked, and having a general level of "no level 1 Bidoofs in exchange for level 100 Darkrais".

I see it as this, you usually see horrible offers because the good ones are taken really easily.

I find the GTS to be half-half. It's full of hacks and ridiculous offers made for the purpose of cloning, but I did get my first Shuckle, which had Pokérus and good IVs (as far as I can remember), through it, and it's also how I got all the starters. It's somewhat useful, but to be what it was meant to be it needs obvious improvements.

The GTS is very good for getting Pokemon you normally can't obtain. I have a very special Pokemon not many know about.

It's okay, but I find it sad that I have to use a hack checker after I trade. xP

its pretty good I think it was one of the best features to help one to complete the Pokédex

I love the GTS itself. It's just the *****bags that use it are unreasonable. (remember that time when everyone wanted a Regigigas lvl. 10 and under? WTF?)

It was good in thought, but in execution no so much. It's too easy to abuse and put up ridiculous trades that make it near impossible to find a good trade.

It's a good addition to the games. It's how I got my Infernape.

How else could I have gotten my Lvl 9 and under regigigas before platinum legitimately?

Its good and bad at the same time. Its good because you can get Pokemon that would normally be harder for you to get easily (such as the starters), but its bad because you can't request/search for Pokemon you haven't seen.

That makes it a lot harder to complete Pokedex's if someone (such as myself) didn't go to earlier events (such as Deoxys or Arceus).