I recently procured a Lumia 900 to give Windows phone a spin.
I've used every iPhone, and nearly every GSM android available in the US, and the culmination of the experience leads me to this thought on the Lumia, Windows Phone, and Microsofts position in the smartphone market:
The Lumia 900 is a very nice phone, built specifically for Windows Phone 7.5 Mango. Any one who has read the spec for Windows Phone Microsoft controls would realize that as far as windows phones go, this is a very nice unit. I find the hardware build solid, voice quality is possibly the best of any smartphone I've used, battery life is good, and the Camera is tapping hard on the door of the iPhone 4S in terms of optical quality.
On to Windows Phone....: The OS is solid IMO. It doesnt have any stuttering issues with the UI, works as advertised, and when compared to the stock experience of many Android devices, it does in fact "smoke" them. The market place has a number of good apps, with some obvious absentees that make it less attractive. I use my phone for business, and I can tell you that if you are using Office 365 (Which includes Exchange, Lync, & Sharepoint) this phone goes quickly to the absolute top of the list. Integration is obviously tight, and well planned. Native versions of Microsoft Office do kill most competitors offerings, and all the things you expect from the Exchange experience are present and accounted for.
Conclusion on WP7.5: Microsoft made a quality tool, that does what it is supposed to and reliably.
My .02 on their position:
Microsoft failed to realize that the Smart Phone market is not driven by business. Its driven by regular consumers. RIM failed to acknowledge that, and given their current failing position you would have thought microsoft would have taken a Que from them. Their "cloud" experience is certainly pale in comparison to Apple or google, and frankly the phone doesnt offer much in the way of wow factor. Home users dont have Office 365, and most people dont even use Skydrive. Apple has taken the consumer market by storm. Microsoft had plenty of real Market observations, and coming in this late in the game they should have been able to create something to capture businesses, and individuals alike. They didnt.
The only advantage Microsoft has is a solidarity in their platform, where as google is woefully missing that. However lack of a decent marketing strategy will waste that advantage, and keep Microsoft competing with RIM for weakest smartphone maker on the market.