Just switched to a Nokia Lumia 800 for my work phone. The 900 would have been a lot nicer, but we're with Telus so 800 it is.
I have to say that so far I'm very impressed. The Windows Phone OS is actually incredibly smooth and straightforward to use. The animations are slick, and I love the live tiles. The Office apps and Outlook mail client are pretty top-notch (so far), so it certainly has the business capabilities we need. I just have to play around with some of the security stuff now (remote wipe, viruses, etc), but I gotta say that overall I'm really having a lot of fun with this thing. I had fun playing around with a few of the Android phones I've had along the way, but when you boil away the wallpapers and widgets, they're really not that different from using an iPhone (which I've been using for about 4 years now). Windows Phone 7 is the first one I've tried that really has a completely unique experience, and Microsoft pulled it off quite well. I've caught myself a few times wondering if Microsoft really did actually create the Metro UI, since it feels so much better than anything I've ever seen from them before... but that's probably a different discussion for a different thread.
As a company, we're starting to toy with the idea of allowing staff to move away from BlackBerry, but as the guy who makes the final decision, I don't feel comfortable having to sport the myriad of Android devices out there (in addition to security concerns), and I don't really feel that iPhones are the way to go yet either (deployment headaches, etc). If this Windows Phone meets our needs on the security side, then I think it'll be the way to go. I really wish Telus would get the 900 though, it sucks to have the one with the smaller screen and no LTE.
Speaking of Telus... I figured out that Nokia has actually released 4 updated versions of Windows Phone 7.5 specifically for the Telus models, which Telus has never applied to their phones, or even passed along to their customers. I had to download a hacked copy of the Nokia Store Tools suite just to install the updated Telus firmware for my Telus phone, because Telus wouldn't give it to me. Some things just never change...