The first Saw had a great twist ending - so great that, even if you saw it coming, it still worked. The sequel, while decent, seemed to have only two goals: be more sadistic than the first and try to have an even more shocking twist. The third installment was obviously made with similar goals in mind. Unfortunately, "the big twist" just isn't. Sure the games are more sadistic this time around, but they are just more of the same that we have seen in recent films (Saw, Saw II, Hostel, Chainsaw: The Beginning, etc.).
I enjoyed the movie, but I felt there was too much time spent revisiting the first two films. Not just revisiting, but rewriting. I won't give specific details here because I don't want to spoil it for anyone, but when you see it, you'll understand. The revisiting and rewriting of the earlier films works as far as the storyline goes - that is, nothing shown in this film is unbelievable or inconsistent with the first two films - it just seemed unnecessary to me.
The film as a whole worked well, and it was a fitting chapter for the series, but I don't think it benefited from the approach the screenwriters took. However, if you just want to see some great sadistic scenes in the "Saw style" and are willing to take it at face value, I think you'll enjoy it.