I build modded original Xboxes and on these Xboxes I run emulators for most of the old game systems. Like Atari 2600, 5200, 7800, 400/800, NES, SNES, N64, Sega Master system, Sega Genesis, Colecovision, Intellivision, and of course MAME. There are a few more, but they are mostly hand held systems like Lynx and Gameboy.
I find the Xbox does an excellent job of playing most games and if you have used the "S" type controller, then you know the controls are pretty good. Not as nice as dedicated controls on a coin-op, but they do work well for most games. They are also totally re-mappable. The only thing Xbox MAME is lacking is support for a trackball. In my opinion its the best way to go to play all the old games, Xbox games, and MAME.