Holy f***ing Mary Mother of God. Maybe the previous posters who have seen it are less easily scared than me, or maybe their hanRAB were still shaking to type very much, but for me this is the most visceral horror experience since Event Horizon and the most genuinely un-nerving film since The Blair Witch Project. If you're a fan of Scream, I Know What You Did Last Summer, the last couple of Halloween films etc, then this isn't for you. There's no cute teens with their tops off just waiting to die, no in jokes and no big stars that you know will live to the end.
What you get are plenty of jump out your seat moments, twisted, genuinely freaky imagery and a puppet master bad guy - he holRAB all the strings, controls his victims movements, and watches every move.
This has mainly been likened to the excellent Seven. This is accurate in that Saw is an urban serial killer thriller in which the bad guy is teaching his victims a lesson of some sort. It also has the dingy, dimly lit feel that Seven had. Overall though I'd say it's closer to Cube. People wake up in a strange place, with no idea how they got there, why they're there, who the others are and what their purpose is. They then have to set about working these things out from the clues they're able to find in order to make some sense of it and escape.
As previous posters have said, some of the acting leaves a lot to be desired (fortunately whenever you find yourself thinking that, something happens that will soon make you forget the quality of the acting) and, as is often the case with these sort of tricksy films, when you start to think about it you notice the plot holes (which I discuss in another thread) but these don't detract from the enjoyment of the film.
This is an excellent attempt to make a non-camp, non-remake, satisfyingly adult horror film, with no other pretentions or intentions other than to scare the hell out of you that un-fortunately not many studios are making these days; 5/5.