Hello, new member here. I actually have a GP2X, ever since it first came out. I've always wanted a mame cab, but playing those older games on the handheld unit (and on my TV with the TV out) has convinced me I REALLY am serious about it. My wife also plays the GP2X sometimes, and she now wants a mame cab as well!
Anyways, GP2X is a great system with a great community behind it. I get nearly flawless gameplay from old DOS games like Jazz Jackrabbit and Commander Keen, Ultima 7, Doom, Quake, Descent, Heretic, and most of the old Sierra games like Sam and Max, Monkey Island, Full Throttle, etc. Also play all my favorite console games like Genesis, SNES, NES, TG-16 (including CD games), as well as other handheld games like Lynx, Game Boy/GB color, Game Gear, and Neo Geo Pocket. The main draw for me though (especially lately) is perfect NeoGeo emulation (plays even the newer games like Rage of Dragons and Samurai Shodown 5), CPS2 emulation that is very playable and improving almost daily, and MAME (plays a somewhat small romset, but plays them well).
I know I am really bragging about the GP2X, but it is worthy.... excellent system! I highly recommend it.