I'm going to get this out of the way, by stating up front what is wrong with this movie:
-The opening credits take WAY too long. What was he thinking? Start the movie, get a beer, come back 5 minutes later.
-Related to #1, the movie is slow to start and the pacing seems a little uneven. Not much happens but talking for a while. If you expect a continuous slash-o-rama, you will be disappointed.
-The premise is a real doozy, and might bounce some people right out of the movie before anything even gets started.
-Like alot of Carpenter movies, this one has a small budget, and it sometimes shows.
-If you don't like John Carpenter's soundtracks, well, this has got one, only more so.
Having said all that, I am forced to admit that "The Prince of Darkness" is one hell of a horror movie. Not quite on par with the damn-near-perfect "The Thing," and in fact a very different kind of horror movie, but a killer movie on its own merits. It is also more thought-provoking and intelligent than almost anything wearing the "horror" label. It makes great use of the "everything you know is wrong" premise to really screw with your head, and if it doesn't freak you out once or twice, you just ain't human.