I think that first off, I should say that I read this series and absolutely loved it. That being said, I also thoroughly enjoyed the movie as well. It was fill with the same amount of comedy, and action as the book, and even though the details were changed there was still the same underlying themes. I think it was a job well done.
So I have seen this movie twice already and it's barely been out for two days...Basically it's an amazing movie. Everything was really well done. I especially loved all of the parts with the high school kids, specifically Mike he was hilarious. Every character was cast well, and even the ones I wasn't sure about surprised me. So my recommendation would be to go see it. It's AWESOME!