I've held one at a shop. I don't like how the buffer pin detent is secured, I'd need spare roll pins should I ever have to replace one. The way that the selector is installed, would make using the FCD selector hard to remove if the trigger or hammer needs to be repaired or a stuck primer removed through disassembly. Yeah I know, I could use what they suggest, but after using the FCD selector, I am definitely not going back. And like another previous poster, not too keen on the unconventional style of takedown pins.
Just my one sampled opinion though, not intentional to be biased, just saying my impressions which by coincidence is not favorable. But I suppose if one is chasing weight to the point where function isn't as much of a concern but negligible weight is, then this would be awesome I guess.