I keep hearing good things from friends in New York City who've already seen it. More importantly, they echo almost word for word the sentiments of this reviewer:
For me, Mamet is a real hit and miss director. While I generally enjoy his films, I find that at times his stylized dialogue comes off as precious and pretentious. I absolutely hated his film, HEIST, and I thought that STATE & MAIN, while cute, was wildly over praised. I was, however, a huge SPARTAN fan, and I'm happy to say that REDBELT is almost (but not quite) as good as SPARTAN.
Mamet's dialogue, while still very distinctive, is a lot more naturalistic than usual, and helping matters is the fact that he has an absolutely first rate cast delivering it. Chiwetel Ejiofor is quickly establishing himself as one of the best actors of his generation, and he gives a terrific performance as Mike Terry- our conflicted, but stoic and pure hero. Not only can he spout off Mamet dialogue like it's the most natural thing in the world, but he also comes off very well during the (few) fight scenes. Supposedly Ejiofor has no previous martial arts training, but this is not obvious at all while watching the film.
If this ends up being half as good as Spartan I'll be impressed.