Before you read this, note that I had read the book before watching the movie.
At first, I was like... Dude, no. They completely changed the story line. They changed the main character, gave her a completely different background, even key points about her ([spoiler]especially when they said her brother had died, which in the book doesn't happen, instead she fell asleep inside her car in the garage, while the car was still running, which ended up on her being intoxicated by all the gasses[/spoiler]). [spoiler] Even her first dare was completely different, and her best friend Sydney din't actually sign up for the game in the book.[/spoiler]
However, as the movie went on, I thanked the directors for changing the course completely. It got even better than the book. The outcome was amazing, in my point of view. Emma Roberts and Dave Franco were the best cast I could ask for for these two, and it's nice seeing them playing roles completely different than what I was used to.
The only thing that disappointed me was [spoiler] that while the book has an open ending, which might mean that if the writer ever wants to do it, she can come back and write a sequel, this movie ends with no prospect for a second movie. Which isn't necessarily bad, but I liked this so much that I wouldn't mind a second one. [/spoiler]