They are a "hobbiest" quality gun. I wouldn't want to trust one for serious work. They are not mil-spec and don't strive to be. A much better gun can be had for a couple hundred dollars more. They are plenty good for plinking and fun and I'm sure they could serve as a defensive weapon but like I said there are better guns. Mission drives the gear.
I'd try to stay away from Olympic Arms, DPMS, Bushmaster and a couple other odd ball brands. A pretty good mil-spec carbine is Palmetto State Armory and they can be had for less than other top of the line guns like Colt, Daniel Defense, LMT and BCM.