I've been through Bioshock 3 times. Love it every time. Something about the electric plasmid and the wrench just does it for me. You can get it for $30 new this week at CC. A definite buy at this price if you haven't played it. Sure, it'll go down eventually, but it's definitely worth its original retail. At half that it's a no-brainer.
Assassin's Creed is $30 everywhere now, I think...$25 or so used. It's fun. As other folks have said, it's repetitive...ridiculously so if you're going for all the achievements since a few are just "collect a billion flags" or "rescue a zillion innocents from hoodlums". Either way, it's worth a play because it's still very well done and the play mechanics are more fluid than any game I've ever seen. Simply running around cities climbing and jumping off stuff is almost as much fun as the gameplay itself.