I've written somewhere around 1/3rd of a NES emulator. It does the CPU, and the backround half of the PPU, but not foreground sprites, sound, controllers, or running at decent speed ;)

Anyhow, I found the CPU part a lot of fun, and there's public domain test roms with expected registers after each instruction that you can use to test and confirm. Interaction with other hardware and emulation of that hardware gets a lot more challenging. The gameboy is certainly different than the NES, but I suspect there's a lot of similarity at the high level. It's certainly reasonable to stop (or pause, perhaps), when it stops being so fun, too.

I'm also not sure porting an emulator is going to give you the same understanding as writing one, but maybe.