I have a gp2x, and honestly I'm not thrilled. It's not that it doesn't do homebrew decently, (because it does) it's just that it's a unit that only emulates games from 10+ years ago so it has a limited selection of games, and nothing to look forward since I played everything out 10 years ago and all the years of emulation since.
after a month carrying that bulky, buggy, brick, I switched back to my GBM and bought it a nice 4gig SD card... nothing better than polished, relatively new games. I'd probably recommend emulation on the PSP or DS--not because they emulate better, but because if you get sick of homebrew, there are other things to do with it. though Gp2x does have a certain coolness factor associated with people not knowing what it is.
and if you think psp/ds owners complain about control issues, the gp2x has an 8-way D-pad (vs. standard 4-way) and a wide variety of programmer skill levels. most haven't figured out how to use it, so while you may have a nice fullspeed emulator, getting the stick input to work correctly is iffy. and really, the system doesn't feel that good in the hands to start with.