I've got the original GP2X and UAE4All is brilliant. It's fast enough to play Nebulus and Ruff 'n' Tumble and probably more demanding games. It's really smooth, and probably even better on the faster Wiz.
I also use a SNES emulator and can run Donkey Kong Country at almost full speed. The GP2X is absolutely brillant for emulators before you even get started on the native games.