From the creepy setting to the scary villains, and horny, screaming college kids, this one has it all. The acting and characterizations were competent and believable (for a horror movie), with fine performances from all involved.
Many of the typical horror clichés and dumb moves by the characters to serve the plot and score more victims were absent here. In fact, most of the characters didn't even realize a psychopath was on the loose until late in the film, when it was really too late.
The setting was really milked for all it was worth by the director, with scenes not only throughout the creepy mansion, but a number of them below the house and within the extensive grounds. Some of the blue lighting in the outside sets really lent a spooky, almost supernatural ambiance to several scenes.
I can't think of too many flaws here. Some gore hounds have complained about a lack of blood and guts. Did they see the same movie as me? I recall plenty of violent murders. The camera did not linger on them overly, but neither did it quick-cut away instantly like so many films of today. Some of the dialog was cheesy, but I don't have much of a problem with that if the story is good and the characters reactions to their dire predicament are believable.
Overall, I found this to be a highly enjoyable, terrifying film experience. Certainly one of the most underrated horror movies.