I will admit that I have not played any of the Silent Hill games, but I have seen youtube clips and read up on it. I could see why people would complain so much if they are fans of the game.
Now as a movie watcher, I would have to say that it has a plot that has a habit of changing a little too much. It goes from exploring a town, and running from monsters, to a battle against fanatics. As well as how Bean's presence in the film seemed a little unnecessary and added too much time to the movie.
Now, that said, this movie does fall under the category of great special effects. Just seeing the town rot away into the Dark World and then rebuild itself is one of my favorite effects, because it gives the impression of hiding the horror beneath the scenery. Also the monsters were well crafted with the right make up and special effects, right out of the video game(if there wasn't a game first, I would say they were borrowing the monster concepts out of Hellraiser).