Wow, this one takes me back. I saw this when it played theaters during my freshman year of college. I liked it but haven't seen it since. I remember that scene of the guy falling out the window looking pretty unconvincing then, and based on the video above it has not aged well. I've never read Peter Straub's original novel, but I'm thinking I need to give a spin.
The Straub novel is better, although very different. I'm not sure how much the script deviates from the novel because there were a lot of scenes that didn't make the final cut (CINEMAFANTASTIQUE's article on the film describes some of the abandoned material).