Got mine today. I figure I'll add my impressions.
Physical build quality is excellent, one of the best handhelds I've used thus far. The buttons sort of feel like Altoids. This is not necessarily a bad thing. Yes, imagine pressing Altoids as buttons and you'll get a general idea for the feel. The buttons do not feel mushy at all either. Another plus.
Sound quality is pretty decent as well, better in comparison to a few audio players I've tried.
Emulators:
GBA: Best of the set so far, still has hiccups on certain games, such as Mario Golf. Metroid Zero Mission works great though.
Neo Geo: Runs at about 80% - 95% speed depending on the game.
CPS1/CPS2: Tolerable, but like the rest, could definitely use improvement.
NES: It runs great. Are you really surprised? Castlevania III hiccups though.
SNES/Genesis: Hahahahahahaha.
Nothing that hasn't already been said.
A few minor quirks I've noticed.
The Y button problem, yeah, it's in pretty much all the emulators. I've remapped to X and A in the meantime.
One that hasn't been posted though, and maybe it's just happening with me. Unless you unmount it using 'safely remove hardware' it will not fully unmount, leaving the device in a 'USB Mode' state until you hard reset the unit using the little reset hole. Pain in the ass. Even with the safe method above, it likes to hang at times. Definitely needs to be addressed.
Sorry for the formatting or run-ons. Quick and dirty post.
Quick Clarify: There are two methods I know of off the top of my head to unmounting USB devices. One is 'safely remove hardware' the other is to right click said drive and go to 'unmount / eject' or whatever. I usually use the method from my computer. This is what causes the device to hang.