My GP2X's primary function is as my Boredom Prevention Unit (BPU). Anytime I expect to be sitting on my ass doing nothing-- bus rides, lying in bed, waiting for a movie to start .. etc. I keep it with me and it never fails to keep me amused. Most of what I do is play games on it-- a combination of emulators and classic FPS'es (Doom, Quake, Wolf3D.)
I play a LOT of MegaDrive/Genesis with DrMDx, especially the Sonic, Ecco and Vectorman games, plus of course the sheer awesome that is Gunstar Heroes. On the NES front, the Mario series, Kirby, Kung Fu, Captain Skyhawk, Guerilla War and Armadillo get a lot of playing time. I don't play SNES games that often, but when I do, I play the hell out of some Super Mario World and Megaman X1-3, also Sparkster. NeoGeo emulation gets a lot of use as well-- mostly Metal Slug 1/2/X, Blazing Star, Strikers 1945 Plus and Puzzle Bobble/Bust a Move. There's also GB emulation; I play a lot of Super Mario Land 2, Quarth, some Tetris Plus and a bit of Pokemon Red/Blue.
I look forward to GBA emulation so I can throw Kirby: Nightmare in Dreamland, the Klonoa and Megaman Zero series on here. They'll fit well.
For multimedia purposes, my '2x is my primary music player away from my desktop PC. My 1GB SD card holds a few hours of music with lots of room to spare for everything else, and that's usually all I need access to at one time- not 4GB+ like most dedicated players (or god forbid, 40GB+.) I play the occasional TV episode or movie on there, too. One of the most invaluable features is that it can view photos from my SD card-- and my card is intercompatible with my digital camera! AND my laptop's internal reader! The camera and '2x can both use the same NiMH AAs too. And yes, it's a good machine for porn, I'm not going to lie to you.
Of course, anyone who's been around a while should know I'm a developer; I spend a lot of time programming for the machine. It's a damn powerful platform, and we're achieving some amazing things with it. I'm really looking forward to releasing som more info about me and the rest of the team's projects soon!