Not scary at all
I've seen Blair Witch and Paranormal Activity 1 and enjoyed them both for their skill at creating a sense of scary suspense.
Just saw Ghostwatch yesterday. Maybe it's because the horror documentary era is already in full swing, or maybe because I'm not as easily frightened anymore, but I didn't find Ghostwatch scary in the least. The execution was laughably obvious and the scare attempts were too familiar to be effective.
I've noticed that many of the posters who found it scary were pre-teens at the time they saw it. That, and the fact that the faux horror documentary was a relatively new genre at the time likely explains why it was convincing to them. I saw Ghostwatch alone in the dark and was completely untouched by it.