I have a mixed opinion on the GTS. I was very lucky enough to find a shiny Eevee which i traded a Darkrai (also GTS obtained so no loss there) for it and the next day the hotel member's card thing started on the wifi so i could get my own legit Darkrai anyway.
This Eevee i speak of seems legit and is in a normal pokeball and had a 29 spdef iv so it's an Umbreon now.
However i have recieved a hacked Manaphy from the wonderful AAAAAA person (avoid this username) and my Fiance got a Honchkrow apparently caught in the distortion world at the same level Giratina is there lmao, never even mind the fact you cannot capture murkrow or honchkrow in Platinum but the DISTORTION WORLD? hahaha it certainly makes for an interesting pokemon even if it is hacked.
The number of people that ask for lv 9 and under mews really clutter up the place, i know there are thousands of other offers yet we always seem to get these lv 9 and under mew seekers possibly cloning.
Going back to the good points. The last two trades i done were for a Jirachi and Lugia. I put up a spare event Regigigas, the one that unlocks the Regi trio in Plat and i got a legit UT Jirachi, I thank that person immensly as there were quite a few AAAAAA Jirachis there and then i put my Shaymin obtained with Oak's Letter up and recieved a lv 70 Lugia clearly from HG, trained a little bit however my ev reducing berries sorted it out... not only is it legit but it is CALM natured, Perfect for it's best stat!
Oh and the lost Shaymin? No worries from me as i already had an event one obtained directly from a game store with the cherish ball, ribbon and everything.
So a mixed bag, sometimes something reasonable and good appears, a shiny Horsea appeared asking for a female Espeon... my only Espeon happens to be female however she is my precious ev trained Espeon and happens to be the very first poke i've ever ev trianed and she is lv 100 so i passed the opportunity up. There was a point were a shiny Deoxys appeared asking for a Kyogre of all things so i tried to get it but the pokemon was traded to someone else before i could get my foot in.
The most frustrating thing that happened recently before i recieved my Lugia, was that a Lugia appeared and i cannot remember what they wanted for it but i went to trade something else for the thing wanted for the lugia and then for some reason, even though the connection was 3 bars and in the green the connection failed.
A female Charmander i recieved off the GTS bred me the wonderful shiny Charizard i have now.
Anyway there is no point moaning about it because they will not listen and it's still a really cool thing which can prove really useful.