As somebody who has played through the first game, and is working on the second before seeing the movie, I was very pleased. The game doesn't really offer much for the movie to be based off of, so they took many liberties. But I loved every minute of it.
SPOLIER ALERT!
They took the core of the game : Prince finding the Dagger of Time, taking the princess back home (In the game she was Farah, not Tamina), evil guy knows about the sands of time, betrays prince's trust, dad dies, chaos ensues, Prince and Princess fall in love, Princess dies, ends up with the prince (Who is nameless in the games) making time go backward to the beginning and prevents all the bad stuff from happening.
Most of the game is just dungeon exploring, and killing sand monsters..So they had a lot to make up for the movie. But I would love for Warrior Within, Two Thrones, and Forgotten Sands to be made into a movie in the same fashion.