Most are pretty good these days, a couple things to watch out for are used guns that were built. There are a lot of cheap components out there and a lot of guys build them that way and realize they should have put some money into it instead of going cheap. Also, there are a few good companies that put out very cheap versions of the AR, Bushmaster comes to mind. A buddy of mine saw an ad from Cabela's for a $600 AR, I was going anyway so I checked it out. Lower was made of plastic and it felt like a toy. But I have Bushy, it's my close range no optic AR and it's solid, and plenty accurate for it's role. I also have a Remington R-15 which is my long range varmint killer, it has killed dozens of yotes out to 400 yards. Love this gun, accurate as hell and since I'm not a black gun traditionalist I really like the camo.
My plan was to buy a S&W M&P when I got my Bushy, but a guy was selling his Bushy (his wife didn't want it in the house) to Gander Mountain when I was seconds from buying the S&W. I saved $400, ticked off the salesman at Gander and the guy got an extra $100...