Painting your weapon yourself gives it character, makes it yours. Don't waste money for some "picture perfect" paint job, like all ready has been said, the paint jobs that have the shit kicked out of them look best.

I completely recommend disassembly if possible. Paint all the big stuff, save the little crap from paint. It also makes it easier to strip it off, and re-paint properly. Also, painting your gun, and making it "yours", makes it much harder to sell later on when you want to move on, upgrade, etc.