Ok, I bought one of these things. I'm actually planning on giving it to my sister for Christmas as a media streamer.
Maybe I'll post a video of some Mame4Droid gameplay if I get a chance this weekend. It cost $59 shipped from an Ebay seller in Hong Kong and came to my house in about 7 days. My initial thoughts are pretty positive. There are tons of horrible reviews but equally as many positive reviews. The first thing I did was flash a new rom on it. Then I started playing around with XBMC since it is going to be used primarily for media streaming.
It is about the size of a harmonica, plugs into the service USB port on the back of my computer for power and runs Android 4.1.1 Jelly Bean. It has mini-hdmi port, card slot, mini-usb, and usb. Plugging a mouse or keyboard into one of the usb ports works great and I just played one game from the app store with a cheap control pad that I had but didn't take any time to map controls.
I don't think it is ideal for an arcade setup just because there is no really slick way to do it from a software standpoint. I don't think there is an easy way to change android to load an app on startup and hide the system bar etc. Maybe someone smarter than me will run with it though. Anyway, thought I would share. This thing runs XBMC a million times better than my jailbroken ATV2.