I received a "Coleco" brand version the other day (thanks Drewman21).
I've uploaded some PCB scans and a screenshot HERE
The external video output is quite good. The Master System games look as they should, the Game Gear games are scaled to fill most of the screeen.
The internal LCD display is not bad - it's very sharp, bright, and free of motion blur, but not as good as it could have been. The LCD display used is the type usualy found in a video camera's view finder. I'm not sure what the arangement is called, but the subpixels are shifted on each line. The video is scaled to fit the screen with the exception of one game, Sonic Drift II (GG), which has a black border surrounding it.
The sound works - I can't really say much more about it except that they chose a dodgy spot to put the speaker (I often accidentally mute it with my thumb).
Controls are good too, it feels very nintendoish...
Mounted on the PCB is the SMS/GG wonder chip (chip-on-board), a 4MB flash rom (though there seem to be enough address lines for an 8MB chip), some 'glue' logic chips, an audio amplifier, a 3.3v switching voltage regulator, and not much else.
As far as changing the built in games goes... When the system is held in reset (when I hold down the reset button) the address and data lines are held at predefined state (ground), not tristated. This rules out programming the flash in-circuit. The /WE line of the flash is tied to Vcc, not a pin on the SMS/GG chip. This rules out a any kind of secret firmware upgrade mode. To change the software one could remove the flash chip, reprogram it, and solder it back in, or disable the flash chip and solder in another.
Luckily the flash rom is soldered one side of the board and the SMS/GG superchip bonded to the other. This means lots of easy to solder, gold plated vias connecting the two. I've made a diagram labeling all the vias and I'll attempt to solder in a flash rom with some master system code programmed into it.