I have used some for several months. I've used it with a couple different style 9mm bolts. I was skeptical in the plastic ejector and even spoke with the manufacturer about it. They explored the possibility of pinning a metal ejector but the plastic was already so thin that creating a spot to pin a metal ejector made the area that held the pin even weaker. They claim you can get 10,000 or so rounds before the ejector fails, which spread amongst say 5 mags is alot of rounds. I don't see this replacing dedicated lowers, because once you've bought enough mags/adapters you can infinitely surpass the overall cost of a dedicated lower. However, someone that doesn't want to knock out a bolt catch for a full auto lower, or SBR to put in some type of 9mm block it's a great idea.
As far as the manual of arms, they recommend locking the action back with the mag in, then removing the mag, then then visually/tactically inspect the chamber, which is opposite of everything most people have been taught. That can be off putting for some people but for me just tinkering with it isn't a big deal. They offer a CMMG model meant to work with their radial delay blowback system that has its own ejector which acts just like a 566. Basically they cut the ejector off of the mag. CMMG looks like they partnered with them to offer mag combos and complete rifles. I think they're called ARC. https://www.cmmginc.com/9-arc/
I've had about 1,000 rounds with no issues but i've read where people had had issues with certain style bolts. Mine is a PSA i paid $250 for a complete upper. Some (BOLTS) are cut just slightly different than others. Some of them have to "break in" before they work reliably.