The chatter seems mostly positive, and I hate to be a magpul zealot, but if a lower won't take a newer Pmag, it is probably significantly out of spec.
I disagree completely.
Magpul is very clear that their Pmag is sized to the Colt spec magwell. There are in spec AR magwells that are tighter... some more than others. I have some lowers that will drop free all mags, some that won't drop free older Pmags, and a couple that won't drop free even the Rev M Pmag. They are definitely in spec (Spike's Tactical branded), but Magpul's Pmag is on the top end of the STANAG magazine specification. They even had to develop the Emag in order to have products that will work in some of the newer European rifles.
If the Pmags will drop free out of a given rifle, great. If they won't, there's still plenty of other quality choices out there. The Pmag is a good product but it is hardly the end all be all of all magazine choices, nor is it the standard by which even lowers ought to be judged.