My two CMMG's run great.

As far as uses, good for training, plinking - they're perfect for introducing new shooters to the sport - converted AR, or dedicated upper both run great, low recoil, huge smile factor bringing people into shooting.

The dedicated uppers have some very distinct advantages if you have a limited number of AR's, and my recommendation is still for that, but if you feel compelled to train with the exact setup of your 5.56 rig, the convo kits can be more cost effective.