Bing + yahoo now have just over half of google's marketshare (edit: in the US):
http://searchengineland.com/bing-reaches-20-percent-search-m...
Anecdotally, I've noticed my campaigns getting a better CTR on Bing (perhaps their audience is less tech-savvy or just clicks on more ads?), and the CPC is lower to boot. If your market is a good fit for their audience, I consider advertising on bing to be a well-kept secret right now.