well, first let's get one thing out the way - this was about £130 for a device that's pretty much a pi3 in a DMG gameboy case. Now, around the web a custom Gameboy with new screen and internals (PiZero usually) runs between £350 to £400 and they're in great demand so this is an absolute steal.
And it's actually very good, but has some flaws. I shall list them:
The case is solid, very well put together and looks pretty great but aesthetically it's a tad short vertically; the original GameBoy was a little taller. As such it feels a tiny bit "top-heavy" such that it would fall forwards out of your hands. The controls are in the same place, so there's not a lot of room for your palms to grip it at the bottom. I would have preferred it to be basically a DMG case, which would have left room for a bigger battery and maybe an analogue stick too. Not that there's anything wrong with the battery - I get 4 to 5 hours even running N64/PS1 games so it's fine. But more would have been nice, and a bigger batter might have given it more heft.
The controls are also fine. The buttons are very good, better than the Pandora's. The DPad however is not. Firstly it is very "clicky" and needs quite a lot of force especially in the left direction. Secondly it's recessed into a cross-shape cutout in the case, which makes diagonals a little tricky until you get used to it. The read-facing shoulder buttons are badly placed - they would be fine if the case were taller as mentioned above, as you wouldn't be clawing your fingers to get to them. But again, you can get used to it (I have).
The software is Ok.. Obviously the creator of the 'Stone has no control over it, but it really lacks polish especially when it comes to actually viewing game lists and terminals etc.
Which brings me to my last gripe: The screen. It's... awesome for gaming, awful (REALLY awful) for maintenance/terminal work. It's a 320x240 LCD connected via Composite. For gaming, that's neat and retro-looking as you get CRT-like artefacts and most older games were created to be viewed on pretty crappy portables so there's no issue. For games that were intended to run on their own LCD display (GBA et al) then text can become a little hard to read. I found text in Ms PacMan Maze Madness virtually impossible to read.
And for maintenance work - RetrorangePi, the distro in question, demands a minimum of a 640x480 display, and so is filtered down to 320x240. That's not a big deal for the main system as you can use a theme with a larger font, but it's hell at the command line.
Fortunately it does have HDMI out, so using that on a nice HD display with a USB keyboard and mouse is the way to get things done.
I actually love it. Obviously it's massively faster than my Pandora, and small quibbles with the form factor and screen aside I've been playing it for about a week non-stop. And it plays everything up to and including N64/PS1 (and loads of Arcades, natch) without even breaking a sweat.