I have bought two J&T kits, a few upper receivers (A3 "flattops" to replace A1 and A2 uppers), and a couple of barrels, bolt carriers, and other asst parts. All have been top notch. In one of the kits I got someone else's missing detent spring.
The J&T upper rcvr finishes match the DPMS lowers perfectly.
More importantly, they go BANG!!! every time, no malfunctions of any type.