I wrote 2 games for the Gameboy/Gameboy Advance back in the day (Hot Wheels Stunt Track Driver and Matchbox Emergency Patrol) and it was an awesome experience. There's nothing like being able to hold the entire machine in your head and have every line of code executing being written by you.

If you're interested in development I suggest using the No Cash GB/GBA emulator; it's spot on and only had one glitch that Nintendo found in testing due to something about switching off the screen at the wrong time. I did almost all of my professional development in it and only had to burn to a Bung cartridge (used to be available from HK for cheap) for final testing infrequently. Much cheaper and probably better than the official dev kits available (but I wouldn't know, I never used one).