I built a J&T kit gun. Great, no problems.
100% functionality.
Chambering appears to be for milspec 5.56mm as opposed to tight 223 sporting chamber which seems to occasionally cause problems on other kit guns...
However, the default setup uses a std non-chrome-lined barrel. I'd recommend - unless you really need to save money - upgrading to a Colt or Bushmaster chrome-lined barrel as they're much easier to clean, last much longer, and corrosion isn't really a worry.
Also you might wanna upgrade your buffer. The std J&T buffer is a black plastic thingy filled with BBs/shot. Works fine, but the std style buffer is the way to go. Only a few $$ difference.
I would go for J&T esp when compared to other kitgun places (like Sherluk, Model1 Sales, ASA, or Oly Arms). Service is excellent, good quality, and Jesse & crew stand by their word.
-Bill Wiese
-San Mateo, CA