Bing Webmaster Tools is pretty much essential for any SEO analyst. Not all SEO optimizations carry over between Google and Bing and it's good to get up-front validation on crawl errors from the search engine itself. Bing Webmaster Tools is especially relevant now that Yahoo and Bing are under the same roof.
Very usable overall, quick to find information and conduct analysis for example of how a price of content is performing on Bing search. User interface is a bit clunky at times but easy enough once you get used to it.
Bing, which is free to use, is a good adjunct to our use of all these tools. If I had to stop using all but two, Bing and Google would be the ones that I would keep. Others have their place in our cycle based on what Google and Bing are reporting, but these are our early warning systems. Even if they don't offer the best way to a solution, they are the final word in what appears via their search index.