I liked this movie a lot.
I've never played the games, so I dunno how well it mimics the games, but this is the first movie based on a game where it actually FEELS like it's based on a game. Everything down to the general atmosphere of the movie, to the camera angles, to the music. The camera angles and general cinematography of it all was very good. And the music was awesome.
I understood most of the story. The ending was the only thing that confused me. I'm guessing that it's just meant for interpretation, and if so cool. But I still kinda would have liked an explination as to wtf. So no. You don't have to play the games to understand the movie.
This movie was not very gory except for only a few parts. And it really wasn't scary. It was definitally creepy as hell and very much a thriller, but it wasn't jump out of your seat scary. It really felt more like an action/thriller, and I thought that was really neat.
All the actors were good (I also thought Rose was pretty cute), but the damn cop was an awful actor.
There were some parts of the movie where I joked with my friend.
I thought for sure, all the monsters were CGI. But it turned out, that aside from one or two monsters (the beetles, and zombie baby creatures), all the weird creatures were actors. The make up department and actors that were in the costumes were spectacular. Though I wish they would have given Pyramid Head a bit more screen time, they didn't over use him which was nice. Because he was pretty much only there for fanservice of the games, but I can assure you. Any time he WAS on screen, he had the most powerful presence.
Oh, and the climax of the film is NOT for the faint of heart. Fair warning. it's rather gruesome.
Overall, I thought this movie was awesome. Not perfect, but I'm not a real movie critic, so I don't expect perfection.