Saw it earlier this evening. Here's my two cents. Note that I'm speaking as an avid fan of the games, so...there will be a bit of nit-picking on my part.
First off, the visuals are absolutely spectacular. Amazing. Perfectly captures the feel and look of Silent Hill. The transition into the Otherworld is spectacular, and the monsters were incredibly well done. I also thought the acting was pretty damn good. Rose sounded genuinely terrified throughout the film. A few awkard lines, but nothing too bad.
Now for the complaints... Story was a bit of a departure from the games. I wish they either stuck to a straight adaptation of one of the games, or went with something completely original. The mix of both in this case was not very well done. Some very important aspects of the fanatic order were lost as a result, and that includes a lot of Alessa's backstory.
Also, way too many people, and too few monsters. In the Silent Hill games, each volume contains just a handful of people... There were almost 60 or 70 in the church, and that really destroyed the sensation of isolation and depair that is so common in the games.
Back to the monsters for a short bit, I didn't feel they were used very well at all. The new monster, the Janitor felt unnecessary, and the almighty Pyramid Head could have had a much stronger role. The Nurses being all clustered together, while making for a tense scene, was really bizzare. And Rose didn't fight ANY of them at all. Not even some good old lead pipe action!
So...yeah, while it wasn't bad by any stretch, I felt it could have been a good deal better. If a sequel is made, I hope they'll stick more to the source material, or at least follow the themes of the series more faithfully.