If you haven't had the opportunity to try a Windows Phone 7 device in person, I have to say that you are missing out. Today I had the chance to play with a prototype device from Samsung, and frankly, I was blown away. The phone is really fast, and everything feels consistent from app to app. The "metro" style is also very refreshing -- it's heavy on typography and flat UI elements.
As a developer that has been using XAML for a long time, making powerful and beautiful applications is extremely doable as well. The flexibility Silverlight gives developers and designers isn't matched on any mobile platform to date.
There may be stuff missing from the initial version of WP7 when compared to other platforms like Android and iPhone, but what it does have is done very well. Compared to my Nexus One (which has similar hardware), the operating system is very responsive, and animations are very smooth. I'm not saying that Android is unbearably slow, but compared to Froyo, WP7 takes the cake.
If Microsoft plays their cards right, they could be a force to reckon with. What do you think of Microsoft's chances?