I did not feel Silent Hill was a terrible movie, but I was disappointed with it. Sure, the effects were amazing. The atmosphere was fantastic. Pyramid Head looked thoroughly badass, yet his addition was purely fanservice. He serves practically no purpose in the plot other than being another demonic creature that emerges from the Otherworld of Silent Hill. Also, the movie was never genuinely scarey. Some parts were creepy, but never truly disturbing. Some scenes are nearly direct recreations of scenes from the game, and I felt that actually hurt the movie. How can you be scared if you know what's coming? They played out like cutscenes rather than parts of an actual narrative.
Speaking of story, the audience never really gets any information about the plot until the last act in a massive exposition scene told entirely in flashbacks and narration. It may work in a game, but it's bad filmmaking 101. I feel that's one of the major hurdles for video game-to-movie adaptations. Narrative devices in games work differently. Pacing is altered. Interaction adds a completely different dynamic to storytelling. When you're directing a character to walk through the darkened hallways of the elementary school in the game, it's essentially YOU navigating those hellish corridors as opposed to passively watching an actor do it. There's a disconnect that needs to be bridged for the transition to be effective. To this date, none have been completely successful.
Acting could have been better if the dialogue weren't so stupid. Cybil, while standing in a devastated building with a massive hole in the floor says, "Something bad happened here." Ya think?
The verdict: If you've played and enjoyed the games, you will probably enjoy, or at least tolerate the film. If not, you will hate this movie and want your money back.