I ran 1 20-rd mag through it with no hic-coughs, and now run exclusively the Korean 150-round dual-snail magazines, and have had ZERO malfunctions in ±4,000 rounds. It's a smokin' pistol !! I use totally the cheapest ammo I can find.
My wife bought me one for Christmas. I love it for what it is. I love the weight. No issues at all thus far. I am in the process of placing a new gas block to allow for iron sight. Just a personal preference.
A buddy of mine tore down his carbon 15 for cleaning and to play with (his first AR) he took the buffer tube off and while trying to screw it back in stripped the threads on the lower with the metal threads on the buffer and plastic threads on the lower. Then trying to force it in he snapped the lower where the buffer screws in. I wasnt there but I dont think he was being careful or knew what he was doing. I since have bought the upper and have no problems with it at all. Thats my experience hope it helps you out.
Just a heads up on anything Bushmaster. Make sure the gas key screws are properly staked. I do not know about the newer models, but the older models had sub standard BCG's. I know from experience.