The Exorcist (iTunes) is one of those classic movies that is often considered a “must-watch” by those that have seen it and is recommended by many horror fans, because of its importance to the genre. Honestly, I can understand why people feel that way, since the movie was pretty creepy (especially for the time) and the special effects were quite awesome. Unfortunately, the actual script for this movie is not terribly impressive. There is barely any plot at all and (unless I missed it, because I was multitasking) I don’t remember them ever explaining why that specific girl was possessed, what the significance of the medallion found in Iraq at the beginning of the movie was (other than the fact that it seemed to be something related to the priesthood), and even how the old priest died (though it was, at least, implied).
If you’re looking for scares, though, rather than quality storytelling, you’d still do well to see this film. I’ve also read that the Director’s Extended Cut (which I didn’t see, because it’s not on Netflix Instant) has some additional, extra-creepy scenes, so watch that version if you can.