I have two for a couple of lightweight builds. From .22 LR to .458 SOCOM, so far so good. PROs for me is their light weight. Cost might be a pro for some, but I got them for the weight reduction, period. Cons: One, I do not like the molded in trigger guard. You can change that if you are good with files and saws and a Dremel. Otherwise you are stuck with it. Two, and this has supposedly been corrected, the pivot pin can come out completely quite easily. To remove the pivot pin you take it out about 1/4 inch and rotate it 90 degrees. It then can be withdrawn completely. Well, you can do it inadvertently also. Mine pulled completely out when taking the uppers off. The fix was simple enough for me, a steel detent and a steel pivot pin. End of story. NFA has addressed this issue with an upgrade but I am not familiar with what their fix is. Personally I did not like the trigger and it was heavier than advertised. Both broke at 5 3/4 pounds, not the 4.5 advertised (I think it was 4.5, might have been 4.75). Anyway, I did not like the way it sounded (petty I know) and all my other rifles use RR NM 2S triggers so I dropped one in these lowers also. Other than not liking the trigger (but it worked and was very crisp with no creep or over travel) and the pivot pin coming out, I have no complaints. It makes for a very lightweight build with my 5.56 with 16 inch Wilson LW barrel, MI 15 inch Gen 2 SS handguard and MP CTR stock and mil-spec VLTOR buffer tube, topped off with TROY BUIS sights it comes in with a 20R Lancer AWM mag at 6.66 pounds.
The .458 SOCOM should be lighter since it will be the similar profiled barrel but with a .458 hole in it.
I am sure there will be more in the evolution of polymer based AR lowers, but it seems so far that NFA has fixed some of the issues the PC lowers had. We'll see how they do in the long run and if there are shortcomings, I'm sure well see another polymer lower by someone else or a redesign of these. Either way I think they are here to stay.