Igorrock, that made me think of some additional questions: What's your exposure to these things? Are they common or legal for hunting? Are they common for sporting use?

Around here, military based models are basically the only game in town! Parts and accessories for these are so common, it's hard to justify anything else, at least in my budget range. I'm not sure I can think of a semi-auto that doesn't have military roots. Caliber conversions are readily available, but I'd say 98+% are .223/556 caliber. The rifles are common for varmint and small predator hunting. They aren't specifically illegal for larger game(deer), but there are laws about minimum caliber. No common AR round meets these laws, but the rifle it's self would be legal. The larger frame AR, the AR10, could easily accommodate a legal round.

As for availability: I'd say most people who have more than one gun, have a MSR-based rifle. I don't know what percentage are skillful with their rifle and what percentage bought one because they saw it in a movie.