I watched it after reading that it was the "scariest movie ever" and it was very very disappointing. Let alone it not being scary in the least bit, the movie wasn't even a nice watch.
The plotline wasn't as original - there have been numerous movies before about people being trapped inside (The Fog, The Mist, etc).
And to top it off, the movie didn't explain everything well. There was logical flaws throughout in multiple scenes.
However, I think Sandra Bullock did a good performance but that's probably the only relatively moderate factor in the movie.