the upper is near-milspec. All it is missing is HPT on the barrel (high-pressure testing). It is MPI'd and uses proper barrel steel according to advertisements, although there is apparently controversy over whether or not that is true. No one has come forward with metallurgy test results to disprove CMMG's advertisements, though.
Their carriers are now all staked properly, but their bolts suck. Make sure you replace the bolt with a good one. The bolt is the only real issue with CMMG, that keeps them from being a top-tier AR manufacturer. If they would put better bolts with their gun, then only a select few brands would surpass them for quality versus price (namely, BCM and Daniel Defense).
With that said, I got my CMMG way back when. I got it with a full-auto carrier and asked them to upgrade to LMT's much, much better bolt. I have since replaced the BCG with a BCM one. My CMMG has not ever missed a beat, the barrel stays cool, the machining is ridiculous (especially the feed ramp cuts), finish is great, and it is superbly accurate (under 2 MOA or so with PMC and UMC).
Better buys out there for the money, but with a better selection of bolts, CMMGs will do anything.
The Bargain bins are made of mish-mashes of parts. The barrels are usually nitrided and probably aren't subjected to proper testing. Lower brands are mixed, finish is meh, and they have no warranty on them. However, no one has reported a problem with them. I'd put them on par with something like Del-Ton for being a great value buy.