I've seen the 5th movie twice now, and I thought it was definitely the best of the series. I did notice some of the omissions, but it's not like this is the first movie where they've made some major changes. I thought most of what they changed made narrative sense, and didn't detract from the movie just because it wasn't exactly the same as the book. I definitely think it helps to not read the book for at least a few months before the movie comes out, not so that your standards are lowered, but so you can judge the movie on its own terms.
I thought the acting in this one was the best so far as well. Each of the kids still has awkward moments, but they are really starting to grow into the characters and make them their own. But this movie had two great standout performances. Imelda Staunton as Umbridge was one of the best movie villains I've ever seen- she was like a fluffy pink cobra. And the girl who played Luna Lovegood stole every scene she was in. This was also the first film in which I've particularly liked Michael Gambon as Dumbledore, and I'm even more excited to see Alan Rickman get to cut loose a bit more in the next film, when Snape will ahve a lot more to do.
So basically, I liked it.
And BTW...I get that these are essentially kids' books (although some of that perception is due to marketing), but I feel sorry for anyone who assumes they can't or shouldn't enjoy something simply because of prejudice against the source. Also, the first book came out ten years ago, so many of us adults who enjoy them now were kids when we started reading the series. Are we all supposed to stop enjoying them just because we entered some arbitrary age of maturity?