I got my MPX8 and have been playing with it a lot. Haven't been able to get the "editor" software working on Linux but I don't think I'll even bother to figure it out since making kits on the device is so easy.
SOUND: sounds great, reverb effect is very tasteful but effects the volume a LOT, maybe a slight bass boost at the outputs, "Tune" parameter is limited to +/- 4 semitones which is pretty weak
PLAYABILITY: pads have a really comfortable texture on top, extremely responsive, all the flashy colors are fun
EDITING: jog wheel plus four buttons, if you've ever used an electronic instrument from the 80s you'll be right at home (so yeah, pretty much everyone on this site), screen is big and not overly bright and is actually very impressive
SD CARD STORAGE: the only complaint here is that you can't have any folder structure AT ALL, all samples have to be in the root directory of the card
MIDI: haven't tried it yet
I expect this device to get some serious firmware upgrades. At $99 it's going to make it's way into a lot of people's hands and they're gonna want some extra juice. Fingers crossed that Akai doesn't just immediately jump to the next upgraded model and forget about this little guy.