Got my Lumia 800 a few days ago (thanks Nokia!). In general the hardware is pretty decent with an amazing screen, and a solid industrial design. Major quibble is the charge port is under a hatch which strikes me as completely pointless given the device has to be charged every day.
OS wise, Windows Phone 7.x has a very impressive and sleek UI but it's not hard to see the cracks after using it a bit. For some reason Microsoft has chosen to make all menus lower case which really looks stupid given that they're not even consistent in doing it, e.g. WiFi is capitalized and surrounded by other menus which are not. Apps and settings are also scattered around the UI so you actually have to leave an app in order to change the settings for the app which is plain dumb (Apple does this too). My biggest annoyance is if an app wants to download something (e.g. a 100MB map in Nokia Drive app) then you must endure the download because if you flip away the download is suspended. Its the most frustrating and silly flaw borne from a fundamentally broken single tasking interface.
The market place is also afflicted by a dearth of decent apps, many of which cost more money than their Android / Apple equivalents. On the positive side, games are handled in a far more integrated way than Apple or Android with XBox Live being put to good use and offering trials for lots of games. I was surprised not to see Skype in the market place store.
So in summary hardware good, software so-so. I can see users who are intimidated by Android might feel more comfortable with this OS, but really it doesn't offer any compelling features of its own. Given the price of the device I would not recommend this phone to someone who has a choice to get something running another OS.
One thing it has piqued in me is a desire to get my existing Android code to run on it. Best as I can think I'll have to compile as much of the Java code with GWT and house the lot inside an HTML view or phonegap wrapper. Should be interesting to see what happens.