I actually enjoyed this a lot. It is not your average horror flick, and it does have some elements of surprise in it as well. The characters are nicely played and fits well into this movie. It has a lot of great gore scenes and some good humor too. The monsters are not too bad either. Even though there are no explanation to them, it still does not matter, as the movie just circles around the characters, and their battle to survive... which in fact never gets boring.
All in all I can definitely recommend this for horror buffs, especially if you are looking for something more than just "Hi, I'm Candy, I'm a teenager, and I like to go down to basements and get naked before I get killed by the big scary serial killer" kindda movies.
Well worth my time. I can only hope the followup will be as good as this.