Not terribly suspenseful, scary, or unpredictable, "Possession" is, at best, a half-decent watch when you're bored and have nothing else to watch. Because it had both Sarah Michelle Gellar and Lee Pace in it, I gave it a go, but it took two sittings to watch the entire movie, as it just didn't capture enough of my interest to make it through the whole thing the first time I sat down to watch it, which was a shame, because I like both the lead actors, and that's the only reason I bothered to come back and finish the movie.
If not for them, I wouldn't have bothered, as I already figured it out early on, and the ending was just stupid.