I have a Windows Phone now (in face, I'm posting with it right now). It's insanely nice so far. Unlike my experience with Android, I have more complaints about the handset (an HTC HD7 in my case) than the underlying OS. It has the first onscreen keyboard that I've used that even compares to the one in iOS, and it is better in a couple of ways and worse in others (namely, the autocomplete doesn't do as much handholding, which is both good and bad). The interface itself is very different from any other smartphone I've used, but it doesn't feel different for the sake of being different -- it's actually very functional.
In some ways it takes the best of both worlds. Instead of widgets, ala Android, or traditional icons, it uses Live Tiles, which allow you to start an app and also get at-a-glance information. For instance, a weather app might display the current temperature and conditions on its tile.
It also has very nice integration with Xbox Live, allowing you to chat with friends and even earn achievements by playing mobile Xbox Live games on the phone.
Probably the weakest point that I've seen so far is the web browser, as it's a hybrid of the IE7 and IE8 browser engines. On the plus side, it feels snappy, keeping up well with Safari on my iTouch.
I his platform will get to be very nice as it matures. For a new phone OS, I've been very impressed thus far.