Where / when did AT&T promise monthly updates? I don't think MS even releases monthly updates to the slow ring, let alone for non-insider phones.
Ever since build 10572 back in October, the one with hugely improved performance, I think fast ring builds have easily been stable enough to run on a daily driver. I've only had one random reboot on my 950, which has always had fast ring builds, and I'm personally enjoying much improved battery life with the latest 10586.71 fast ring build. The current 10586 builds are a far cry from the old 10511 and so forth days, where it was an unreliable mess of crashes and lag. Just my $0.02.