I like how they changed it just enough to make it a little different but still ended up being the same story as the book. I had no idea that the villain was the same as in the book (I don't want to spoil the movie for anyone by saying who it is, although chances are if you saw it or want to see it you have read the book or are familiar enough with the classics). They hide it well with the whole Phantom thing. I especially like the way they made Jekyll have to ingest the formula like Stevenson intended instead of making him change whenever he got agitated like in the Hulk. I suspect they changed it not to be true to the character but to distance themselves from the Hulk movie out at the same time. It made it seem more of a curse that he had to choose to change.
I did not like the addition of Sawyer to the team. I don't know if he is in volume 2 of the book but he was useless in the film. He had the whole father son dynamic with Quartermain but who cares. I know they needed a token american to bring in the girls. Although I liked Mina's abilities they were not true to the book. I would have liked to have seen her with a little less power since she didn't end up using it anyway. I mean the scene with the bats, she could have just climbed the walls and taken each attacker out herself in a cool vampiric way. The final confrontation with the betrayer (no names) was good, she had all the power I wanted to see.
Overall it was a good adaption of the book and that is really what I want in a comic book movie. The action was good and the story was full but not complicated. This is the kind of film that both kinds of people can enjoy: comic and non-comic fans.