I spent several hours this week investigating Windows Phone 7 and overall I was very impressed but for some major issues I have with the browser.
I was very impressed with the user interface; it is simple, clean and had very nice transitions user interface tweaks. Also the development environment is very well thought out; the screen design is defined in XML and C# runs behind it, all very smooth and developer friendly.
It's a bit of a shame, the browser is good and runs well its just the webkit directives I use so much do not work. I read today Microsoft are investing $400M on developers to ensure at launch there are lots of apps and games supported. Microsoft you should think about spending a small amount on browser compatability with webkit; it's only a small number of things you have to add/change; I could give you a list if you asked me... but it is really important that it is compatible with the rest of the industry.