I know a couple bipolar girls (friends). They have mood swings, be prepared for phonecalls filled with laughing or crying. Otherwise, I find them to be extremely loyal. Their insecurities tend to make them grab on to things they like with both hands. If she falls for you, you'd be set for life. So long as you don't mind the strong emotions, I'd say go for it.
Plus, if she goes split personality someday, two girlfriends! Awesome!