They're solid, nothing terribly special. That has more to do with BCGs as a whole though, if they function well and do not visibly degrade the rifles potential accuracy. What more can you hope for?
They have two models the nanocoat (which is pretty damn expensive) and the standard which is in line for what it is. They are both contracted from elsewhere though I do not recall who makes them.
In the scale I rate them A+ function, but if we're honest they are in the expensive compared to other comparable.
Current BCGs I have in current guns:
LMT, Daniel Defense, Geissele, Young Manufacturing, BCM, Colt, FN, Toolcraft, Aero, YHM, Brownells (both Chrome and Nitride) and recently picked up a KAC and PRI set for special projects.
I mention this because If I'm honest the only lemons I've gotten for any of the companies above have ALL (three in total with most recent being two months ago) been from Colt in the last 3 years, but prior to their purchase.
Otherwise they have all been fantastic.