Quoting from a rather dated review by Preston Gralla which may still be somewhat indicative:
"For its features, customization options and openness, Android has no peer. The downside is that Android can be rough around the edges, and the exact feature set and implementation you get -- not to mention which release of Android you get -- are subject to the whims and control of device manufacturers and service providers.
"If you're looking for the most elegant, simplest-to-use phone with the best integration of hardware and software and with the biggest number of apps to choose from, you'll likely opt for iOS and the iPhone. But you'll give up the ability to completely customize your phone and apps, and you'll subject yourself to Apple's rules about what is allowed to run on your phone.
"If Microsoft software and services are the center of your world, Windows Phone 7 is an excellent choice. But if you want to be able to choose from a wide variety of apps that do remarkable things, then Windows Phone 7 isn't the platform for you.
"Carrier choice will also likely play a role in which mobile OS you select. If you want the widest range of carriers with the widest range of price points and feature mixes, then you'll want Android. With iOS and the iPhone, the only carriers to choose from are Verizon and AT&T. If you go with Windows Phone 7, you'll have more carriers to choose from than you would with iOS (Verizon is expected to introduce a Windows Phone 7 device this month), but far fewer than you would with Android."
"With phones based on Google's Android operating system and Apple's iOS commanding nearly 90 percent of all smartphone sales, some might ask why Microsoft keeps trying. Apparently, the company feels the smartphone can be done better. And maybe it's right. According to our survey respondents, Microsoft's Windows Phone 8 clearly bests Apple and Android."
If it's the best, how come nobody buys it?
One explanation springs to mind: As it often does, Microsoft has piled-on with spambot responses to the survey.