Well,
I've had my GP2X for a week now and I LOVE the thing. If you are into emulation like I am there is nothing better that I've tried. I was running an overclocked PDA but the battery life is terrible, the buttons are poorly placed, and only a few select emus worked. I've played:
NES- works just fine, no probs at all
Lynx- works well, some games don't load, maybe bad roms, overall very solid
SMS/SGG- perfect, no probs
Genesis- AWESOME! (using Picodrive), my genny emu for my PDA wouldn't load half the games, this one is superb, the cream of the crop!
AtariST- hit and miss, when a game loads it plays well, some games will not load but will on Steem (an atarist emu for PC), but having something very similar to a handheld Amiga is pretty awesome!
SNES- not bad, many titles work well, I do crank up the GP2X to 266mhz, some titles have sound that is a bit "off", some titles don't load (seems like any really big rom more than 4 megs will not load), but I was kind of anticipating a poor emu and it's quite good actually, remember to overclock
Intelly- poor, this one surprised me, most roms will not load, if they do then some do not have any sound, then ALL my sound is cut out of the GP2X and I have to reboot the system, only seems that there is one emu for this console
NeoGeo Pocket- great emu, hard to figure out how to get it to work, documentation is pretty non-existent, thank you GunPei!
I haven't tried NeoGeo or MAME yet. Seems a bit tougher than some of the other emus to get started. I will though. And I havent even started GB/GBC. That will be shortly.
Overall, I'm VERY impressed with this little handheld!