Definitely an improvement over street view, especially with regards to verifying map information. The images, however, are not so seamless. Check out these two or adjoining areas - notice how the angle changes.
Well, they're useful for different things. For one thing, it's usually hard to read street-level signage on Street view, but it's nearly impossible except for the biggest big-box signs using Bing's birds eye.
I go back and forth amongst Bing's aerial and bird's eye views, and Google's satellite and street views. They each tell their own useful story, and each have different date of recording. You never know from one to the next which will have the most current data. It's especially disconcerting when adjacent bird's eye views in a construction zone are from different flights... hey, where'd that building go!
But amongst them, what a savings in field work time!... not that time on the ground for truly up-to-date information isn't still essential. But it sure helps when field work is impossible or outside the budget, or for preliminary passes even when field work is part of the project