The first saw movie was actually a pretty decent horror film. The second one was great for its comedic value (especially if you watch the actors commentary). It's gotten exponentially worse/more hilarious since then, and I have seen all of them out of morbid curiosity. I'm not proud of this but it's the truth.
A lot of people say the worst thing about saw is that it's torture porn, but really it's the main villain never shutting up about how clever he is in really stupid ways. It's a textbook example of what happens when writers try to write characters smarter than themselves.
I thought we had passed Saw 8 already and that the franchise was up into the double digits by now, with entries like Saw XVIII: Jigsaw in Space and Saw XXI: The Search for Jigsaw's Gold. Or at least that's what it feels like, that there's been a bajillion of these awful films made in the past few years.
You forgot about the Saw/Hostel crossover (Sawstel) and Saw XIII:Jigsaw in Hell, and the ever classic Jigsaw vs. Freddy vs. Jason. Saw: Resurrection got a little played out, though, with the whole voodoo possession ritual thing. It also seemed kind of weird that he'd want to possess Dolph Lundgren.