Not sure if you've used WP7 phone yet or not, but I have and the UI and the experience it gives to the everyday joe's is much better than Maemo5/Symbian^3 imo. That's quite a feat for something so new in terms of UI. It is silky smooth and is very pretty. What many non-geeks really like. Heck, even I was impressed how "nice" everything looked and felt. Of course, it's quite limited for me and many others here however. =P

But the new Mango update will make WP7 very competant and in many ways, better than iOS/Android. It also feels more complete than Android due to its simplicity and common look. Android has so many features but feels quite inconsistent in many areas imo.

I reckon it has a good chance to be a major mobile OS.