I think it's a great movie, and watch it fairly regularly. The film is all but incomprehensible, but the lack of any coherent plot/story can create a dream like quality that you never know may have been intentional.
Visually there isn't much to fault; there are a couple of exceptionally well designed shots, and some breathtaking sequences. I think the film's depiction of Rasalom is brilliant, and avoids some of the more Hammer Horror, Christopher-Lee-in-a-toupee aspects of the novel. Scott Glenn makes a good Glaeken, he looks ancient and weary and tough all at the same time, though through Mann's lacklustre direction he does come across as slightly wooden most of the time.
The Keep itself is very well designed, and it is how I picture the Keep whenever I re-read the novel.
The story will disappoint you and the plot will confuse you, and the ending will surprise you (primarily because of the rather abrupt way it stops, as well as the liberties Mann took with the story) but not even being knowledgeable of the novel will help you. The film is simply nonsense if you try to follow it, but it is very atmospheric and beautiful nonsense!!