I've only done a single trade so far on the GTS. And guess what I wanted? It's an Eevee! After all there are seven Eeveelutions, so to catch 'em all I need seven Eevees.
I figured the starter Pokémon are always wanted so I just started up a new game of Pearl, got myself that silly fire monkey (I hate monkeys ) and as soon as possible put it up for trade after transferring it over to Diamond. The next day I had a new Eevee - awful stats and everything but it was good enough to fill up my Dex.
I think it was a good trade, trade a Pokémon you get in 5 minutes for one that you need to play for 25 hours to finally get.