The police are given a choice as they investigate him. If they follow his instructions they'll live. If they don't, they won't. They usually don't.
Given that premise, this story has been repeated in one form or another over 4 sequels, though Saw 4 wasn't that bad a slasher movie. Because Saw 4 was decent I bought this movie. It's not good. They have all your standard "Saw" ploys in the film, but it's too formulaic and anyone who has seen any of the previous films will know what'll happen here.
If you can put up with that, by all means watch this movie, if you can't don't bother seeing it, it's not worth the money.