I just saw the Silent Hill movie last night, and I'd definitely recommend it! Even if you haven't played the game, it's still a great movie. In fact, you'd probably like the movie better if you haven't played the game yet.
Personally, I liked the game a lot better. The movie just couldn't grab all of the puzzle elements that the game had. Other than that, I thought they did a great job! The casting was absolutely perfect, and it really felt a lot like the game. There were a few cheesy parts where they tried to make it look too much like the game, but for the most part, they did a good job. It was surprisingly gory though....
As I mentioned, the story worked well whether or not you played the game. There's really this kind of trade off on whether you play the game first or see the movie. If you play the game first, I can almost garrentee you'll like it better than the movie, and the movie will lose some points. But if you watch the movie first, you probably wont have as much fun with the game. Either way, you should at some point play the game! The first 3 of them were very good (Silent Hill 2 was my favorite).
PS: For anyone else who wants to dicuss it, make sure that you make a clear warning to people if you plan to talk about spoilers. It's a nice courtessy to those who haven't yet seen it.