I own an HD7 (after 8 months with an Evo 4G), and I'm very impressed with WP7. I love the UI, and the whole thing just seems like the ultimate smartphone OS (unlike Android, which is more like an "everything but the kitchen sink" mobile OS).
IMO, WP7's biggest selling points are gaming and music. It's the only phone compatible with Zune Pass. It has Xbox Live integration and even has some games that interact with their Xbox 360 counterparts (Crackdown 2 and Fable 3 are the only ones ATM, but I'm sure there will be more in the future).
WP7 also has two major downsides:
1. no turn-by-turn navigation. You can still view maps and get directions, though. This is coming with the Mango update later this year.
2. the web browser isn't so great as far as rendering speed. Thankfully, zooming and scrolling are still buttery smooth, much smoother than any Android device I've seen. IE9 is coming with the Mango update, which, from what MS has shown, looks to be the fastest and most capable smartphone web browser yet.