I have one of these GameBoy Advanced in my hands now (yes, an import). Its a pretty nice system (only Mario at the moment, will get US Zelda since I can't read Japanese), but I don't like the triggers at the top -- they are a little uncomfortable.
GBA will definitely own the portable market. This thing is more powerful than a SNES! Its a 32 bit system that can handle all the SNES roms from a long time ago (minus the buttons, yes I know), so there is potential library of games to choose from, plus all the Gameboy games and color Gameboy games. If you like the handhelds, the GBA is for you.
I just wish the GBA launched like the original Gameboy. Headphones, batteries, and Tetris (with a plastic cover). The only real complaint about the GBA games is the lack of a plastic cover for the games. Other than that, its a solid unit.
vash