I am not quite sure why I went to see the new Saw movie. I basically hated the first Saw mainly due to some of the actors being incredibly annoying and the story being way too unrealistic. I am not sure if I liked Saw II and Saw III any better. I am quite sure I didn't like Saw IV, but still I won't be surprised at all, if I end up watching Saw V.
Saw IV is more or less exactly like the previous movies. You get the puzzles and some fairly explicit gore and some nicely done torture machinery. You also get one hell of a freakin' lot of flashbacks and many an attempt to link the action in Saw IV with the previous films. You also get some new characters who obviously gets killed for the most part and you meet a lot of previous characters in the loop.
Jigsaw and Amanda died in Saw III, but still the saga goes on and the puzzles and killings continue. While I still get fascinated and enjoy watching the cruel machinery in motion, I have to admit that I was actually kinda bored after about an hour or so. The movie is not exactly a long movie (93 minutes), but you don't really care too much who lives or dies in the end, since the characters are basically just dull or annoying or both. It is a great shame that Kerry (Dina Meyer) was killed in Saw III, because with her in the movie at least there would have been someone sexy to watch. But I guess she got bored as well, and asked to be slaughtered to get out of this endless loop.
At the end of the day you'll probably talk yourself into watching Saw IV if you enjoyed watching Saw I, II and III. Just don't expect anything new, fresh or different. The story is completely unrealistic and doesn't make too much sense. But that is nothing new either.