WP7 has hit the Microsoft campus on November 18. This was the date when the free phones the company gave to all the employees have arrived.
I've got one, too, and has been playing with it ever since. So far I cannot imagine going back to my iPhone 3GS.
The UI is slick and extremely responsive. In two weeks of use there was not a single case when the UI lagged. Not one frame of scrolling, zooming, or moving between apps. Not a trace of hesitation. It is also quite intuitive, everything you use often is at your fingertips when you need it.
Search is fully integrated into the experience. It works in the context of the currently running program - in maps it searches maps, in IE it searches the Internet, in Zune UI it searches the music, and in the Marketplace it searches the app store. It works well, too - the results are formatted for the phone, and the way local results are mixed in is uniquely useful for a mobile device.
The IE works great. Really, truly great. It feels faster than the iPhone's Safari browser. With the Samsung Focus' gigantic, bright screen it feels like a mini-tablet. When I bought the iPhone I noticed that I go to my laptop to browse less. With this device I go back to a real computer even less.
One of the reasons that I was awaiting WP7 was that the iPhone was syncing my calendar unreliably. Some of the appointments were transferred, and some were not. I have heard similar complaints from my peers who are also very heavy calendar users. Needless to say, this is the most critical part of the device for me, possibly even more important than the actual phone - I use it as a phone at most twice a day, but I use it as a calendar and email device at least once an hour.
Both calendar and email sync flawlessly on WP7. So does the rest of Office integration. I've spent a few hours reading a book formatted as a Word file, and, again, performance was fantastic and the document rendered flawlessly.
The device is rock solid. I have never - ever - had to reboot it for anything other than the change of the system locale.
Of course, as a v1 device, WP7 has a number of problems as well.
First, it has no way to upload custom ring tones to the device. Yes, this is actually true - the only ringtones that are available are ones preinstalled by the OEM. I have no idea how they managed to cut a feature that at this point is available anywhere else.
Second, the phone only supports a handful of mostly western European locales. For example, it has no support for entering Cyrillic, which, with me being in Russia right now is a big problem. Actually, this plus the absense of copy-paste makes entering an internet search query in any language other than English impossible. Given iPhone's completely universal keyboard that allows writing in any language with ease, this is a big disadvantage for anyone who speaks more than one language.
Third, the phone does not support - this is absolutely, amazingly stupid - user names that have spaces. So if your company has a "First Last" template for the user names, you are out of luck - Exchange sync will not work. Whoever tested the user input dialog - please hang your head in shame now.
Windows Phone 7 lacks the iPhone's breadth of apps, but this is actually an opportunity for developers, and the gap will probably be closed quickly.
Other than that - the device is actually great. I gave away my 3GS after playing with WP7 for less than a day, and 2 weeks in I did not regret that decision for a second. Very highly recommended!