I bought a Windows Phone 7 device. Loved it. Still the best mobile UI I've experienced. It had a ton of smart ideas too, like integrating different messaging services into a central app - totally breaking up the app-centric model of iOS and Android.
...except they didn't provide any hooks for third party apps to integrate. Said it was coming. Along with multitasking and other stuff that was already common in other OSes. It never did. Instead they announced Windows Phone 8, and stated that almost all WP7 devices would be ineligible for an upgrade, thus burning their relationship with early adopters. I believe they did the same with some devices from WP8 to 10. Nowadays they've given up everything that made WP unique, so there's no need to use it.
Sad thing is, their initial timing was great. Google had dropped the ball on Android - developing an entire version specifically for tablets (3.0) while the phone OS floundered. Compared to Android 2.3, Windows Phone was a breath of fresh air. But by the time they'd decided my phone was EOL less than a year later, Android 4 was out. I switched back with no regrets. Don't get me wrong, it would have been an insane uphill climb for anyone, but MS, with their resources and XBox etc., if anyone could have entered the market, it could have been them.