As someone who has sold plenty of them with warnings here's my take. At the Gander Mountain I work at part time they are certainly the most unreliable AR. In fact we take them back more than all others combined, while there sales are mid pack. As far as I can tell their are four issues.
1) Thin barrel that over heats with heavy use causing groups to open up.
2) Flex in the upper and in the joints between the uppers and lowers, this can cause some jamming issues
3) Ammo sensitivity. Despite the fact they are rated for 5.56 DON'T DO IT. Almost every repair we've had with parts failure had run 5.56, run .223 only.
4) Flex in the upper causes the Picatany rail to loosen, it is only connected with two allen screws.
My conclusion: They aren't a bad gun for plinking and coyote hunting for the money. Don't buy one for three gun, precision long range shooting, a build your going to deck out, or as HD gun, they just aren't reliable enough. I would go with the Smith Sport or the Stag for about $100 more as an alternative. Having said this I'm not anti-Bushmaster, I own an old M4 bushy and it's a great gun, and we have very few issues with their other products. But the C-15 is a different animal, cheap, and you get what you pay for.