I don't have one, but I considered it previously. It's a great phone if you're really into social media. The live tiles feature is pretty cool, and really underrated, in my opinion. Instead of manually checking for Facebook/Twitter/whatever updates, the live tiles automatically update with new stuff. So all you really need to do is look at the home screen and the live tiles, instead of launching each app separately.
Also, the Zune player interface is something that I've always liked. Very simple and elegant. The Zune Pass is a great deal. It's subscription music. You pay a monthly fee, but you get to keep 10 free tracks per month. And you get an FM tuner, which is pretty handy at the gym sometimes.
The Xbox integration is nice. But I don't have an Xbox, so that's not really a selling point.
MS Office integration is nice. I personally don't feel the need to write up Word documents on a little 4" screen. But it's nice to have the option. The email and contact management is pretty top notch of course. Android is kinda lacking in that department.
Also, WP7 phones are tied into your Windows Live account. So you can store/sync all your photos and video online for free. I don't remember if it was 5GB or unlimited.
You should probably not get one, if you're app crazy. Because there just aren't that many apps right now
. Probably won't be any for a while either.
Overall, I think WP7 is pretty cool. But I didn't get one because all the phones are carrier branded/locked right now. I'm going to wait a little longer until unlocked/unbranded models start showing up.