Breetai's completely right.
The game is really, really fucking good, but its just too damned short to warrant a purchase. There's just not much re-playability unless you wanted to do it on every difficulty setting. The survival mode is fun for about 5 minutes.
With that said.....you should rent it. Definitely. The storyline is awesome, gameplay rules, and the ending rules, especially the song they use in the background. I've so graciously put it up on my webspace for all to download, since I love it so...
Poets of the Fall - Late Goodbye