Yeah I think they should have left Silent Hill's history alone. It made sense and was established with fans, there wasn't any reason to change it, I don't understand why they did. The visuals were dead on for most of the film, I think the art team did a great job with that.
I thought the first half of it was good, there were moments that I really liked. I thought the second half of it was downright terrible though. Almost everything after they enter the church was just completely wrong. Team Silent would never have put that many people in one scene, and the end boss looked like something out of fucking Hellraiser. And they explained too much of what was going on; they dumbed it down for american audiences, the games wouldn't have done that.
I feel like it lacked the subtlety of the games. A lot of the horror in the games was implied..you'd find the remains of some terrible event, but you'd never see it happen. In the movie they were ripping people up left and right, especially at the end, it just got to be gratuitous.