I liked the first one -- like juniorgrrl said, because it was funny and it did something new. The second one just tried to re-make the first movie with slight alterations, and in my opinion it just wasn't as funny . . . it lacked the element of surprise that the first movie had. And of course, back in the day when the first movie came out, a lot of the stuff that they were showing (sexual innuendoes and grossout humor) was new and even a little bit edgy -- now it's par for the course.
And of course, half of the best part of the second movie was that they managed to bring back all of the original cast.
They could only bring back about half of them for the third movie, and it even seems like they pretty much wrote the script around who they could bring back and who they couldn't, which means that I don't have very high hopes for it. Maybe they'll surprise me, though.