I agree with JohnHem, I have used yahoo and I believe that yahoo and bing are combined work together and certainly didnt have any complaints about the service.
I actually like the service. I had about $250 in my account and made a good deal off my ads. I guess it REALLY depends on your site and what your selling.
Today Yahoo and Bing together cover about 30% of market. But some keywords won't work in Bing (yahoo) that do work in Google and vice versa. Bing does provide great webmaster tools for analysis