The first Saw movie was amazing, IMHO. It was all set in the one trap, and hardly anything was explained. As the movie went on you thought you understood everything, then BAM!!! the twist smacks you in the face, hard. The art style was very fitting, and the gore was done very well; not too much, and not too little.
The second Saw was the "worst" one, but it was still fairly good. It explained a little more of Jigsaw, although it was pretty obvious that Amanda was working with Jigsaw (spoiler)
Saw 3, despite the mixed reviews, was really good, albeit different than the first two. The complexity of it all rocked, and the acting was good (for the most part) There was more gore in this film, but still nothing too terrible.
All in all, I love the Saw series, and I have high hopes for the new one(s). I think that as long as they remain good movies I'll watch as many as they come out with, and that's what I've come to expect from the creators.