I have to say I liked the movie. This is the one book in the series I haven't read, so I had no real expectations of what was going on. I think the plot was well done, I was able to follow it fairly well. I thought all the actors did a great job. I actually enjoyed the flying scene, its a nice way to show the disparity in his life at Hogwarts and at home. I especially liked the way Sirius and Lupin were able to display their bonds of friendship, and Lupin was able to show his bond with Harry without alot of exposition.
The scenes in Harry's bedroom at Hogwarts helped the place to feel real.
The transitions worked very well as noted before. However, most people are correct that without reading the book you can get the impression that Lupin knew about the map and perhaps had used it, but not that he was one of the people mentioned.
Things I didn't like:
Peter was stupidly played I thought.
The time-turner had enough clues earlier, but I think it's a bit powerful to have never been used again. Does it get destroyed at the end of the book?
Nice to see Token from South Park getting some work.
Small touches I did like: The beans, the wanted posters, the guy disappearing the beer bottle when cleaning the table, the school ignoring the ghosts like they were used to them. the Boggart, the huge executioner's axe.