If you consider the Windows Phone line to be merely the next iteration of Windows Mobile, then, yes, thus far it has been a complete failure.
But I don't think that's a very holistic viewpoint. Despite how terrible Windows Mobile was, it had a good hold of the market until the iPhone came along. It wasn't really a "failure" until it started declining in 2008. Microsoft's response, Windows Phone 7, was a complete reboot of their mobile line, and we have yet to see how it will perform in the long run.
But I don't think that's a very holistic viewpoint. Despite how terrible Windows Mobile was, it had a good hold of the market until the iPhone came along. It wasn't really a "failure" until it started declining in 2008. Microsoft's response, Windows Phone 7, was a complete reboot of their mobile line, and we have yet to see how it will perform in the long run.