Bioshock 2 is definitely worth getting (story's not as good as the first, but the gameplay is better) - but as you say, it's only a good deal if there's a couple of games you're interested in.
Personally, I like to have a couple of cheapo games put aside - to try them out when there's nothing to play. Even if I don't really like it I'll generally get a few hours out of a game - and there's always the chance you'll find an absolute gem and get hundreds of hours - Perfect Dark Zero and Star Ocean being two examples for me.