Most films based on games are pants and nothing more than random bits of fan-service cobbled together into something the studios think will appeal to non-gamers as well. Result is usually a mess that pleases no-one.
However, I'm pretty good at switching my brain off and have always enjoyed the Resident Evil films. It's a good example of a series that gets progressively worse with each outing, but as a zombie film fan I can still watch them and be entertained! The first Tomb Raider was a decent bit of mindless fun as well. Final Fantasy: The Spirits Within was also good although it always get slated and isn't really anything to do with any of the games.
Silent Hill and Prince of Persia are probably the best of the bunch, in fact POP is a pretty damn good fantasy adventure.