I use a Windows Phone with great success, the LG Optimus 7. Has my Xbox live account integrated so Live games contribute to my profile. Everything is integrated nicely. Only annoyance is the camera, but that is hardware, not software.
The problem as I see it, it's hard to break the back of Apple without something that is entirely different. My partner is looking for a phone with a good camera, the only thing stopping me from suggesting the Nokia PureView is the lack of Windows Phone as the OS.
The OS is fantastic, it has never been a problem for me and it's very quick. The problem, as always, is the lack of hardware. Google get around this by supporting everyone, Apple get around this by controlling it themselves. Microsoft is in the weakest position by putting all the eggs in with Nokia.
I would pay to see what would happen if they put Windows Phone OS onto the iPhone hardware.