The movie was far FAR better than I expected. From the beginning, I was hooked. The opening scene was... frightening. I'm all for horror movies. In fact, it's my favorite genre, but this movie (from the beginning!) was truly scary.
After the opening scene it kinda dragged on for a short while, but the concept and ideas behind the movie were truly interesting and there were some surprising and not-so-easily-guessed twists throughout the plot.
Is there a down side? Of course there is. There is a down side to every movie. Like I said before, after the opening scene, the movie kinda dragged on and at some points it was almost difficult to understand. And some things aren't thoroughly explained and leave some small (but annoying) loose ends. Sequel (to the prequel...)? Hmm, probably.
WARNING! There is some violence and gore throughout random and unexpected parts of the movie and it's realistic and straight-forward. And there's some 'bad' but oddly amusing language at some points. I don't want to give away anything so I'll just leave it at this: beware.
FINAL THOUGHT: Great tribute to Linda Blair (please stay for the whole movie to see what I mean), but be prepared to wet yourself (or at least cringe) when some of the more gory scenes come up.