I only ever used the GTS once.. and didn't get anything that I wanted. I even put in a Level 35 Shiny Charmeleon, and all I wanted was a level 10 and up Turtwip. Nothing. That was also when I first started playing, so I didn't know that, ultimately.. I was screwing myself, if that trade even went through.
I would like to get involved in Negotiation trading over GTS, but I need a little more experience with these games before I do something like that.. namely, learning more of the Pokemon, which are best for what, having planned out EV teams with movesets, etc etc..
For the most part, I'll only be getting involved in trades with people from this Community, because I've gathered from the few that I've spoken directly to, and those that have added their own marks to posts I've made.. this community is an honest bunch.. so I know I won't be getting screwed.
Anyway.. that's my two cents: Didn't like my first experiences, but open to trying again once I learn more, and know what to look for.