I purchased a 9mm JR Carbine back in December. I think it's a good carbine. It's very accurate as far as I can tell. It points easy and handles well. The quality of the materials and fit seems good - particularly the barrel.
I did have problems with it it initially though - it would slip into full auto mode at times. I figured something was up with the trigger and immediately reached out for a service repair and had the trigger group replaced in short order. I've put maybe 1000 rounds through it since without any issues (with any ammo I could find ie not the best). I'd have pumped allot more rounds through it if I could find ammo. I do plan on installing an after-market trigger though because I do not like the trigger pull - it's creepy.
When it was malfunctioning I inadvertently was able to experience what it's like to fire a machine gun. Interestingly, it remained accurate when it slipped into full-auto and my groups remained tight.
A couple of hints for good operation:
Don't slam the mag up into the mag well to load it. It's designed so that you slide the mag up (you'll feel it pass the catch and keep going) and then gently tug down to seat it.
I was kind of anal and broke the barrel in by first cleaning it, shooting a couple of rounds, cleaning, shooting more rounds, cleaning etc. I don't regret it.
I use only ball ammo for a couple of reasons; it's all I can find; it's cheaper; I don't really plan on shooting anyone; and my guess is that my lack of issues and smooth functioning is partly because I'm using ball.
Good luck with it and have fun!