Sorry, Sarah, but, since your `Buffy days,' your film career has hardly been glittering. What we have here is a short - and pretty empty - horror/thriller. If you believe the marketing on the poster, the people who made it have something to do with both The Grudge and The Ring. I can assure you... it's not even nearly as good as either of them.
In my opinion, it has two major problems: the first is that it's rushed. It's only eighty minutes long and everything has to happen really quickly to fit it all in. Buffy is now married to `nice brother,' but `nasty brother' is generally hanging around with them. Then, one fateful day, nice brother and nasty brother accidentally crash their cars into each other (yeah, seriously), leaving them both fighting for life side by side in their hospital beds.
Then, nasty brother wakes up, only he claims to be nice brother. And now we have our film. Only we don't. Seeing as it's nice brother in nasty brother, he behaves very well and doesn't do anything nasty. So... who's really in there? Nice brother, or nasty brother? Um, do we care? He's nice, so there's absolutely no real sense of danger or dramatic tension (my second fault with the film).
If you're a Buffy/Sarah fan like me, you'll probably agree that she's the best bit. She does her best, but the script is lame and she could do with either having to fight someone, or be chased by something (ala The Grudge).
Sadly, Possession offers nothing you haven't seen before, only better. Stick to the Grudge or the Ring. Like I say... sorry, Sarah.