An HTC representative, who didn't want to be named, noted that he's been using Windows Phone 7 for several months. He believes the Exchange integration is excellent, and the apps are powerful.
In some apps, there will be three little dots that appear in the bottom right corner. That's your indication that there are settings available to be adjusted. These dots appear in a black bar at the bottom of the page. Sometimes the black bar has other controls, which are sometimes labeled and sometimes not, depending on the app and the way the phone is held. This slight inconsistency might lead to a little confusion.
One really odd thing I noticed is that the screen only rotates to landscape view in some of the applications, but not nearly all. For example, from the home screen, if you hold the phone sideways, the icons don't rotate. They all face the portrait way. Sometimes, apps will rotate once you dig deeper into them, but not on the external menus of it. It's a bit inconsistent and weird. This is something I hope Microsoft fixes.
The browser (IE) performed well enough. Double-tap to zoom in or zoom out, and multi-touch are supported. Again, however, if you re-orient the phone from portrait to landscape, you lose the all the navigation controls for the browser, which makes it less functional.
The Windows Phone Marketplace was active on the devices, but the offerings were bare. Microsoft has said that there will be apps when the platform becomes available to general consumers. Navigating the games, apps, utilities, etc, though, was easy enough. The menus are very graphically rich and colorful.
Native apps -- the Tiles and Hubs -- all worked well and loaded almost instantly. They were extremely quick to jump to life and there were no delays. Third-party apps, however, were a lot slower. The demo IMDB client, and demo Facebook apps, for example, took nearly 30 seconds to open, which is just way too slow.
In sum, the new UI feels slick and polished. The architecture and lay-out is completely different from what Windows Mobile users might be used to, and that's a good thing. What's most absent is list after list after list of folders and files to interact with. It's based heavily on graphics, and larger, easy-to-digest groupings. That's all good.
Will it be a good competitor to iOS and Android? Well, it easily surpasses the usability of Symbian, though webOS and RIM OS users might have enough available to them to prevent them from switching.
Microsoft is re-entering a tough market. It's gone a long way to making Windows Phone 7 a better UI.