First hour = Incredible. Last 20 minutes = Ehh, boy.
There are two things about this movie that make it, for lack of a more powerful term, f@#$ing awesome.
1) Olson's performance.
2) The camera work.
There's also one thing that makes the last 20 to 30 minutes total crap.
1) The cliche 'real killer' / 'it was the same person' ending. Ugh.
Do yourself a favor. Watch the first hour or so, and then just make up your own ending.
The camera work and Olson were both great, though, as I mentioned above.