I've had a cheapo-BT 7.62x39 kit for couple months, and just sprung for for an AR-15 performance upper. Why not? I've had limited experience, only about 200 rounds, but haven't yet had one FTF, with the cheap kit. I know that I lucked out, as I was using garbage ammo. But the accuracy is impressive, and I really can't afford to shoot other calibers as much. Shooting my 200rds of 7.62x39 cost me about $40. Shooting the same amount of 6.8 would have cost me well over $200. I'm not getting the accuracy out of it that I do with my 6.8 or 5.56, but my cheap 7.62x39 rig groups a hell of a lot better than my AKs. Then again, I've been shooting the cheapest ammo out of the cheapest upper possible. I expect my AR-15 Performance upper will group better, and paying more than $.15 a round would give me even better performance.
Here's a pic of my rigs.
I've had good luck with the C-Products mags, but have tended to shoot mostly from the 10 rounder.
My 6.8 is still my hunter of choice, but the 7.62x39s are great plinkers. Hell, they'd wreak havoc on medium-sized game too.
I plan on dragging the new rig to the range, and blasting another 200 or so rounds, this weekend. I'll be sure to post a range report.