I used a Windows Phone 7 phone (HTC HD7) for a week as my only phone, and here are some thoughts. The UI, called Metro, is a tremendous accomplishment — nothing like iOS or Android. It's so smooth and airy and almost organic. In contrast, the iPhone feels rigid and sterile. Animations, for example, are even smoother than on the iPhone. There's an extreme emphasis on reducing all unnecessary UI elements, and lavish use of whitespace. If you think iOS is minimalist, prepare to be pleasantly surprised.
Text occupies a primary role in the Metro interface. You'd never think that text can be so beautiful and that it can structure a UI so delightfully rather than just being content placed within a framework formed by other elements. Then there's the full bleed, where content goes to the edge and past it, as if you were looking at a just one part of a UI that's bigger than the screen.
It's authentically digital. I'm sure the Metro designers had the iPhone in mind when they say, "don't try to be what it's NOT". Another great principle is that transitions between screens are as important as the screens themselves. It feels so responsive and alive.
Not that it doesn't have its rough edges. Some widgets look terrible, for example, the toggle switch, or the action buttons at the bottom of the screen. You can't quickly navigate to a contact whose name starts with S, say, in the contacts app. I also found some basic navigation and usability issues. Once I selected an album to play and browsed to other albums and artists, I couldn't return to the playing album, or find out which song was playing. I did notice that the currently playing screen seemed to be the left of the main screen in the music app, but I couldn't find it later. Another time I was able to reach it by pressing the back button from some other app (the home screen, I think).
Metro is also tremendously wasteful of screen space. Everything is unnecessarily big -- the tiles on the home screen, the contacts in your phone list, song titles in the music app... Simple tasks require navigating to multiple screens, and you see less on any single screen. The HD7 I used has a big 4.3-inch screen, but it doesn't feel any bigger than the iPhone, because the UI squanders all that real estate. This is the biggest problem with Windows Phone 7 — it sacrifices too much function in pursuit of form, as elegant as it may be.
Then there are the mundane problems -- the app store wasn't available in my country (India). It's irresponsible to sell a phone for Rs. 30,000 ($660) without enabling the app store. If I can't use apps, I have to use the browser, which is terrible. It's not even touch-optimized in the most basic way -- the combined address/search bar (yay) is too small to hit. I often have to try several times. The browser sucks technically too, with hardly any HTML5 support. And the default search engine, Bing, can't be changed. Which would be okay for me (though I work at Google) if Bing worked well. Unfortunately it doesn't, at least not in India. I used Bing as my default search engine on my desktop for a couple of months, and I almost lost the ability to find things on the internet. I wouldn't buy a phone with such a broken search experience.
Upgrading the phone is also a pain. The sync app is not available outside of the Mac App store, and it's bad and keeps crashing or eating up 100% CPU without doing anything useful for a long time. Then it didn't let me upgrade the OS, telling me that there weren't any updates. After wasting a lot of time on phone with the semi-informed HTC support, the solution turned to be reinstalling the sync app. This is almost like the old and buggy Windows days. If I have to call customer support to figure out how to upgrade my phone, there's a good chance I wouldn't want to buy a Windows Phone 7 device the next time.
Then there are most elementary and frustrating hardware issues that remind us that HTC is not Apple. You need to use the physical button to take a photo, but you need to press it so hard to get it to take the photo that the phone shakes and most of my pictures are blurry. This is so frustrating on such a high-end phone. It reminds us of a major drawback with the non-integrated Windows Phone 7 and Android strategies -- the hardware vendors continue to surprise us in finding new ways to screw up up basic functionality. I can't imagine this problem occurring on an iPhone, which is what I'm returning to, from the elegant yet semi-functional Windows Phone 7 world.