As some other reviewers have said already, this movie actually has a fairly original plot idea.
The basic premise is that a group of people signed up for a reality game show, and are "kidnapped", and then taken to a deserted prison with a nasty history. If they stay the night and complete the tasks given to them by the show producers, they walk away with $20,000 each. The way the idea is portrayed, it actually would make a good "reality" TV show.
Strange things begin to happen, of course, but are they supernatural, or part of the show? Seems like it would be pretty predictable, but it has a few unexpected twists.
Basically this movie uses tension and atmosphere to drive it for the most part, although there is some occasional blood. As for that, let me just say that in some cases there is a lot of gore, in others there is not. The reason for this becomes clear later in the movie. At least to anyone who paid attention. I have to say it surprises me how many people will crap on a decent horror movie just because it doesn't have tons of blood and violence happening every ten minutes. This movie is not a gorror movie, nor is it designed to make you jump. As I said above, it is mostly about tension and atmosphere. To be fair, I have to say the cover for the DVD is very misleading.
It also has decent acting. No Oscar winners here, but neither would any of them get razzie awards. The characters are believable, and I disagree that any of the dialogue was stupid - in the sense that it is perfectly believable dialogue you would expect from the types of characters portrayed. Your average reality show contestant isn't exactly a shakespearean player or professional speaker you know. They say exactly the sorts of things you would expect your average person to say.
On the flip side I will say that there are a few things about it that are kind of lame, but I think some of them fit. The game show uses a clown video to deliver the tasks to the participants, and its pretty stupid, but hey blame that on the game show producers, not on the movie. It isn't intended to be scary, it's just the type of lame crap a reality game show would use. Some of the tasks are kind of stupid too, but again just what you'd expect from a reality game show.
I also do have to say that some of the characters in this movie do seem kind of dumb, although that's par for the course for most horror movies.
The story could have been a bit more detailed, and more history given on the characters, particularly the antagonist. Finally, I agree that the ending wasn't very satisfying.
In summary, if you are looking for a horror movie with lots of gore and/or jumps, this is not it. If you are looking for a story where everything gets explained, and you learn all about the characters to the point where you know their family history (exaggeration), this isn't it either.
But if you are looking for a movie with a decent plot, and some tension and atmosphere, don't let the negative reviews put you off. This movie isn't terrible, it just isn't great either.
