My opinion: They're smaller than they look in the pictures. They are cheaply made and kinda cheesey. I have one. Even the buttons and other controls just feel really cheap.
I have the Dragons Lair machine. Which, honestly, isn't that bad considering the availability of authentic Dragons Lair machines and the trouble of having to get laser disk images if you want to setup a MAME machine, and it has the LED scoreboard on it and it has 1 and 2 and Space Ace on it. It's plug and play, and it's not bad.
If you are looking for a machine that you're not going to play very much that has unique controls or maybe even a light-gun which they seem to be doing now, then maybe it's worth it.
But if you're looking for regular stick and button arcade games you're probably better off just spending money on one of those "real" arcade cabinets that have 10 bazilion games all-in-one.
And if you're a collector of arcade machines, yeah, obviously the Arcade-1up machines don't count.