I've got a PSA AR-9 that I bought as a kit. Paired that up with an Anderson lower and it's been 100% reliable through a few hundred rounds... I wouldn't call that a really great test, but it's all I've shot so far. I have not had a failure to feed or eject yet.
And I noticed that they have the mag well blocks for about fifty bucks right now- really good deal.
I will say that mine rattles a little in the AR lower mag well. Maybe some play is required for reliable feeding- or maybe it's just sloppy. Anyway, there is a little rattle but I've not had any problems yet. Oh- and I've only used the PSA mag. Have not tried any converted UZI or other Colt pattern mags.
Now to play devil's advocate... I bought a NFA (New Frontier, not tax stamp) Glock 9mm lower from
@DoglegArms and I'm impressed with the machining. If you happen to already have a bunch of G18/G17 mags, I'd go that route over the mag well block. After I get the NFA lower done and on a pistol, I'm pretty sure I'll sell off the PSA mag well block/mag.