I've bought a complete M4 upper and 5 barrels from Classic Arms in North Carolina. I think classic had the complete M4s on sale for $440 a couple weeks ago, I thought if I had an FFL I would have bought 10 to resell.
Anyways, my M4 upper has funtioned fine. I got about a 4" group at 100 yards and that's good concidering I couldn't hardly see my front sight or the target (bad eyes). After testing it I have no reservations about useing their products.
On the Classic Arms site in the reviews for the complete kit a few people said they were getting light primer strikes and had to replace the hammer spring, but other than that they all seems happy with it.
Supposedly they are known in the barrel business, a guy posted on here a while back that he worked in a shop that builds custom (accurate) rifles and they use BCA barrels.