After your review of the first movie, I believe I said I wouldn't bother with the sequels, but, maybe if I'm in the right mood, I'll give them a shot. I think you're right about the space between the sequels hurting the success of this franchise. I never thought of it very highly but that was without seeing them.
I'm starting to think that the merit of this franchise will depend on whether the final installment pays off. The longer it goes the more mysteries it throws out and if the filmmakers don't have an adequate solution to it then it may not be worthwhile. While part 2 and 3 were fun, it becomes a series of cliffhangers and eventually you will want some resolution.