WP7 is like a more exciting younger brother of the iPhone. It has a dramatically different UI and a very consistent look almost enforced on apps. It has good battery life compared to Android and things "just work" in a very iPhone way. Downsides are the lack of content (apps, video, etc). The fact that tons of stuff requires an internet connection (goes through Bing). The fact that tons of stuff goes through Live Accounts (X-Box Live, Hot Mail, etc). And the fact that Microsoft has released a tiered app development framework only allowing trusted partners access to certain features, dramatically limiting the competition within the market and almost choking its own platform to death.
I've spent the last few months making the same decision you have (iPhone Vs. Android Vs. WP7) and as much as I like the WP7 UI and the dedicated camera button, I think the ecosystem of WP7 is by far the worst (even considering iPhone is OS X tool chain only).