I played with mine for a few minutes at lunch time, both as a handheld and in the dock connected to my SDTV through AV.
I love the d pad! It's halfway between the NGPC and the Neo CD pads. It has a great feel, but you don't have to move it as far as the Neo CD pad. The face buttons also feel nice, the L and R buttons feel like mush, but those aren't action buttons anyway. The screen looks pretty good, but not as nice as the current generation of handhelds. I'd say it's better than a PSP 1000, but not as good as a PSP 3000 and nowhere near the Vita.
It's the perfect size IMO, a little bigger than the stick. Doesn't feel cheap at all to me. However, there is one huge negative mark for me: The composite AV out on this thing is TERRIBLE. It's dark, with tons of rainbow banding. Worse than any Neo home system I've ever used, and I've had models with the bad AV before. I really hope one of us can figure out how to get RGB/component out of it. Otherwise, the emulation is really good, the sound is good, and I can barely tell it's emulation. Metal Slug slows down exactly like real hardware.
Feels good, man! It is the exact same size as the old style stick, but there are a few differences. The NGX stick has a satin finish, which I prefer to the Neo's original gloss. I'm no fan of the old style stick's buttons, but the NGX's buttons feel pretty good, but not as good as a real microswitched button like my beloved Seimitsus. Still, it's good enough that I probably won't bother swapping them out. The stick itself feels great, almost exactly like the original stick.
I have some more tests I want to do when I get home, like trying the stick with the PS3 and PC to see if they work, and seeing if the HDMI to component converter will work (I don't think it will.) Also, the Ninja Master's card is 100% SD, I want to see if a computer will recognize it.