A lot of the places it looks like there's poor quality coverage - only high-res photos in certain areas. However the same is with Google, so they could work together. However Google Maps is much faster than this Microsoft offering. Not to say it can't be of some use.
The detail isn't as good as Google's best, but the coverage is very good. I've yet to find anywhere in the UK that doesn't have reasonable resolution, and I've been looking at places in rural parts of East Anglia and the East Midlands. GE doesn't even cover places like Leicester, Nottingham, Derby (it does have a box of good resolution around East Midlands Airport, which includes northern Loughborough and southern Derby), but Windows Local Live does, and even has some birds eye views (very detailed views) of Nottingham, including the station. Interestingly, the birds eye views and the normal view of Nottingham are oviously taken at some time apart, since the normal view shows the tram lines under constuction, the viaduct for the station terminus is done, and the depot and earthworks for the section alongside the Robin Hood line, and the Phoenix Park branch, are being worked on, but no track or stops seem to be in place yet. Also, a MML HST in the old "Bambie" Livery is in P1, with a pair of 2 car Turbostars in P3. On the birds eye view though the tram lines are done, with a tram seen at Station Street Terminus, an a reverse foration HST is seen on P5 (at some point, HSTs swapped from using P1 to P5, which they used to use before privatisation and AFAIK before the Turbostars came in), which is in the new livery, other than the south powercar which is in the old livery, and on a view from a different angle, a Meridien can be seen entering or leaving P5. This dates the birds eye views as quite recent.
I've been using it through Flashearth, and not only is it quicker (quicker than GE IMHO), but it's nice to be able to compare the two different sources.