Wow...just wow.
Everything I hoped it'd be and more. Thus far no installment has come reasonably close to SAW III for me, but this was just about as good.
Right from the beginning, every trap has meaning, the way that the dialogue throughout the film pertains to the violence is extremely thought provoking.
I expected to hate William when he was first introduced, but in the end loved his character almost as much as Jeff, I felt bad for him despite the awful things he's done, and really wanted him to make it.
This does the best job of any installment thus far at both having characters their lesson while forcing them to suffer for their sins at the same time.
The health care aspect was a brilliant way to weave in and out of characterization, moral dilemmas, and themes that personally and socially relevant.
The flashbacks feel just as relevant to the story as what's happening currently, Charlie Clouser was at the top of his game coming up with some innovative ways of using the themes from the previous films at exactly the right times, I think this is Tobin Bell's most emotionally charged performance yet, Costas Mandylor gives the most sinister villain performance of the series that had me anxious to see him get what was coming to him (without revealing whether that happened or not), there are some interesting twists building throughout the film that serve a purpose rather than just one or two at the end there are there to be "shocking," and I was completely on the edge of my seat during the tests to see what would happen to these people since I'm almost more invested when the situations directly relate to the transgressions of the victim(s), and they balanced survivors compared to deaths enough to where I really wasn't sure who would make it.
It feels as climactic as SAW III with a sense of finality to the primary characters of the second trilogy that could be continued, but doesn't necessarily need to be unlike the "to be continued..." endings that were more like pauses in the middle from SAW IV and V.
This is the best horror film of 2009 and the second best movie of the year behind only District 9.