Yeah I use the GTS/Negotiations here and there...
This one time I used the negotiation , I traded a shiny pichu/skitty for a LV.100 Illumise and a milotic...( Illumise was hacked so I had to release it =/ ) Worst trades ever lol ; and no I didn't get what I was browsing for =(
GTS can be useful if you're looking for pokemon that you can trade for ; other than that good luck on getting Zekrom/Reshiram LV.9 and under XD
Best I got from the GTS was a UT Hatched Spiritomb , UT Hatched Oshawott , and a Japanese Ditto ( on the 1st attempt of searching for one ) ; all of em are legit :D
I use the Japanese ditto for breeding purposes ( of course... ) and the other 2 for collecting / desire I guess