I actually decided to give it a shot on the X-Box, and while it could've used another six months of bake time, I have to say that I'm glad I played it. The story was great, and when the game worked, it worked REALLY well. Yes, it has its share of bugs, and the flick targeting system is annoying as hell when it's not working right (locking onto things it shouldn't when you're just walking around), but if you're willing to overlook those issues and enjoy the game for its story and its gameplay innovations, it's worth the used game price (or rental fee). The sense of powering up over the span of the game's story is fantastic as well.
I loaned it to a friend after I finished it, and he had pretty much the same impression I did. Both of us enjoy immersive, story-driven games, and I think this one's pros outweigh its cons. I'd like to see the sequels come out (it's a planned trilogy) with a level of polish this game didn't have.
Perhaps the PC version is in better shape. If so, I'd say go grab it. Even if not, frankly I think it's worth the playtime if you enjoy deeper-than-average games.