Personally, like Mr_M, I think it’s gone from tight physchological thriller to a blood and gore fest. I liked the first one, it was a creative, unique, fresh idea. The second one moved a little bit away from the first one, but by the time the third one came about the entire transformation had occurred, removing any intelligence from the concept and just making it an excercise in watching people be tortured. The morality lessons, the clever whodunnit aspect, etc. are all gone by the time you hit Saw III, and Saw IV, well Jigsaw is dead, that should have been the end of it. Now you’re moving into Jason Voorhees territory.
I’ll probably eventually see 5 on DVD long after it comes out, but I have zero expectations for it.