This movie differed most from its book, but for the first 70% of the movie that was a good thing. The movie was humorous yet dark and conveyed most of the important details.
However....
They completely fucked up the last 30% of the movie. I was expecting an epic battle strewn across the ministry of magic. Instead, they shortened and simplified the battle sequence and over emphasized harry's inhabitation by voldemort. They turned the most epic sequence in any of the six books so far into a lame and short section.
Basically, I thought that although they changed a lot, the beginning sections were great. However, they messed up the ending horribly and that pretty much ruined the movie for me.