I never was a user of the GTS in Platinum, mainly because I didn't care that much about trading.

But now, on Soul Silver, it has been my main source for getting my Dragon team up and running.  

Some awesome deals i've got off from it were a Vespiquen for a Garchomp, a Snorlax for a Metagross, a Munchlax for a Lucario, an Aipom for a Spiritomb and now I just traded the Garchomp i've got before (after breeding it for myself) to get a Salamence.

What were your best deals in GTS, if you've been using it? 

Earlier in my game (about the 5th gym) I had a pidgeotto level 30 who was falling behind and was becoming difficult to train. I needed him for fly however, so I went to the GTS to see if I could find a replacement for him.
I decided to search for Fearow, and there was a level 52 Fearow for a level 20 and up dratini - I quickly ran to the game corner and bought a Dratini, before sticking it in the daycare and cycling up and down goldenrod like mad :P
I managed to grab the (now level 20) dratini out of the daycare and make the trade for the awesome Fearow before the offer was accepted.

All of my pokemon in my team except my starter were received through the GTS (I traded Graveller for Graveller and Machoke for Machoke :P)

Mine was a pikachu for a Lvl 3 Lucario )

That was an awesome trade you got there

I tried to do trade evolutions through GTS, but at that time, I didn't find a Kadabra --> Kadabra for quite a while, so I gave up on it and traded with people in here.  

MANUELF said: Mine was a pikachu for a Lvl 3 Lucario )

That was an awesome trade too

Lucario is a pretty powerful pokemon, i'm actually trying to see if I can fit it on my team, on par with Spiritomb, so I can finally tackle the Battle Frontier 

If it's anything like Platinum's or Diamond/Pearl's it's useless.

Honestly, trades are often unfair like a lvl. 7 Metapod for a lvl. 100 Arceus or a lvl 15 Doduo for a level 99 Charmander.

It has the restricitions of only being able to trade for pokemon that you have seen before, which limits it severely.

Other than that, although the majority of trades are ridiculous (A LVL 1 Phanpy for a LVL 100 Mew? Really? >_>), you can still find some awesome trades. 

It's true that idiots like that enjoy clogging up the space with offers such as that, but if you make your search more specific then you are more likely to find a good deal.

I got a dratini level 40 for an abra level 15 :D

I can't seem to get away from those ridiculous trades, I gave up once I managed to get a foreign Pokémon to unlock the Beyond the Sea course.

would love to do some trading, i only need 3 pokemon but unfortunately my DS doesn't support my internet security settings =(