More words: see my Bad Astronomy review.
Overall, I was really impressed with this flick. I won't spoil anything here, but I will say that I wish I hadn't eaten a big ol' beef burrito before going to see it; my stomach was in knots by halfway through. Not from gore or anything, just from pure tension.
In my opinion, this was a truly terrifying movie in the original sense of the word. I was scared. Not sickened, or jumpy, just good and truly scared.
Very cool. Go see it.