I know that after the second film, the movies started deviating more from the books, but I actually thought that the people who made this movie did a pretty good job (though I'm probably biased because it's directed by the guy who directed "A Little Princess," one of my favorite movies). Most of the changes actually worked better for the movie. What works in a book doesn't nessisarily work for a movie, and most of the changes for this movie were more cinematic.
For instance, Harry receiving his new broomstick mysteriously at Christmas worked better for the book, and him receiving it at the end worked better for the movie.
I also liked the use of clocks in this movie, as this movie was including time travel in it. I thought Gary Oldman did very good as Sirius, and the guy who played Lupin, although I didn't think it was good casting at first, actually grew on me, and he did a good job too.
That's not to say the movie is without its flaws:
1. Where's the consistency with the previous movies? Hagrid's house wasn't at the bottom of a hill in the previous movies. And the whomping willow was in the wrong spot.
2. I just going to say it; Micheal Gambon is no Richard Harris.
3. It was never explained why Lupin knew about the map, or why Sirius went to the Shrieking Shack. I wish they had at least had ONE line that explained all this. However, I have a rule that elements from source material that don't make it into an adaptation still happen in the adaptation's continuity unless they are made impossible to happen (and it doesn't work the other way around, so don't get sneaky), so at least I have the book's explanation for that.
Other than that, it was a great adaptation. The book is better, of course, but the movie was very good.