I think the first two are entertaining, even if I do feel kind of embarassed for liking them.
I'd go so far as to say the first is almost legitimately a good suspense flick, held back by the hasty histrionic acting, and a few plot twists too many.
The sequel, well, significantly weaker, but at least they didn't do a straight rehash. Not a good movie by itself, as in you probably had to like the first one to dig it.
The third is terrible, though. Stupid plot, stupid acting, a storyline that apparently thinks it is somewhat deep (a complaint that can't be put against the first two). Even the traps, the most superficial element of enjoyment in these things, were lackluster.
I'll probably end up pirating the fourth movie. Worth noting that I have never paid to see any of these movies. I did get the first two for free on DVD via buy 2 sales, though.