I'm not the best person to answer this but since no one's answered yet..
GBA doesn't of course need to be emulated, since NDS can play gba games itself, so those work perfectly with g6. As for the rest, nes games work pretty good. There's two ways of playing them. You can either use the GBA side nes emulator that comes with g6, which'll slightly flatten the games since GBA's resolution isn't so great. The otherway is to download a homebrew nes emulator for the DS side, which doesn't flatten the games at all. The downside is, that the DS nes emulator doesn't support saves at all (this was the last time I checked, which was atleast two months ago.)
As for Snes emulation, I don't think it's good on any DS card/cart, and it's definately not good on g6. Some games will work fine (pretty few I think) while others won't boot at all and others have serious glitches. Of course this might have changed since I last checked.
Anyway, the general rule of thumb is, that emulation is better on M3 & Supercard. Hopefully you'll get an answer from someone who's a bit more up to date in these things though.