Just got the NGX, so here is quick review.
Screen does not looks as good as Vita/PSP, maybe not even DS. Graphics are kind of muddy and screen is not bright enough.
Controller is ok, but might have been better with a u,d,l,r cross
Select button is now menu button, not sure why they didn't just label it select. In fact there are L1,L2,R1,R1 buttons - no idea what they are for its a NeoGeo. Its like someone took a cheap handheld for some other console and then made it into a Neo Geo console and didn't want to pay the money to remove the extra buttons. I know one controls the screen size, but perhaps they could have made a button just for that.
Console is too wide, should have been a little smaller with a 4:3 screen
SD slot is a bit too deep, makes inserting and removing harder than it needs to be.
Sounds FX in Metal Slug just don't sound right, music seems to be ok. Waterfall in level 1 is like static fall (noise).
Emulation is ok, but not that great. Emulation on PC makes games seem better, on this console it makes them seem worse.
Forces one to recharge in the doc only. Even the DS has a simple AC charger.
USB controller is ok, but plastics have a low end made in China feel, vs. the original controller - high eend made in Japan feel. Its kind of like someone took the original controller and when cost reduction on it.
NG games are great, but perhaps there are other systems one could hack to have portable Metal Slug?
On the bright side I'm sure something from this will be cool, like hackers putting Unibios on it, or protos shipping on SD cards, etc.
All in all I would hold off on buying it as its not that good.
Hopefully it looks better on the TV. I'm going to try that next.