Ahh - the GP32, I've had one for a bit now.
The games for it are all pretty poor and generally suffer from crackily sound and a choppy framerate, however, where the machine really shines is the homebrew stuff. The conversion of wolf3D is really slick, and the Master system and Game gear emulator runs really well.
The fact that you can store loads of roms, MP3's and home-grown games on one 64mb smartmedia card is really cool.
The emulator for the PC engine is also coming along pretty well.
Basically, If you buy one, make sure you get into the home-coded stuff as the official korean games are all a bit shite.
The screen is better (brighter) than the GBA, and the sound output is of a higher quality (although sometimes crackily as I mentioned before). also the design is pretty comfortable - the shoulder buttons are better than the GBA's for a start.
Its a decent console, and Gamepark are actively encouraging home development and the importing of their console, but I doubt it will ever come close to nintendo's success in the portable gaming world.
Hope this helps.