I used a Nokia Lumia with Windows Phone 8. People say that the UI was better than Android or iOS, but I disagree.
The flat UI of Windows Phone was completely unintuitive. In the real world it's many times easy to distinguish objects you can interact with, by shape, texture or touch. With feature phones or remote controls you had hardware buttons.
With PC monitors and touch screens all controls are virtual, you get no 3D or touch feedback, so you have to rely on visual clues and visual patterns.
The flat UI of Windows Phone was pretty bad, providing no visual clues whatsoever, so you where left to touch text randomly on the screen, in the hope that some of it will click.
I see people here giving examples of technically illiterate wives and grandparents successfully using the Windows Phone, but guys, you're kind of missing the forest from the trees.
It's not that hard to learn how to make a phone call, or write an SMS, given that it's an operation that you're doing every couple of hours. You simply have to learn the path from home screen to whatever you want to do. Ask any child and he'll tell you that this is best done by trial and error and it is never a problem for repetitive operations, because we've got good memories.
The problems happens when you interact with an application UI that you've never seen before. And to be honest, even though I've been primarily an iOS user for the past 3 years, the best of the bunch in this regard is Android.
By comparison Android's material design provides intelligent clues about what can be pressed, or about what interaction just happened. And the design of Android applications is pretty flat, in the sense that available options are clearly laid out in front of you, no need to guess or to trigger hidden menus.
My favorite example was RunKeeper for iOS versus Android. The Android version had clear action buttons for starting a race, allowing me to easily select the type, whereas the iOS interface had the options hidden behind a menu that would appear when I tapped on the logo, which was a pretty dumb idea. I'm not even going to mention the dedicated Back button of Android, because people that don't have an Android just don't get it.
But back to Windows Phone, I don't know what you folks have been smoking, but it had terrible UI. And the apps where horrible. Its only saving grace where those Nokia Lumia phones that had a good quality/price ratio, but Android was already dominating the cheap smartphone market and a cheap Android might have been worse, but at least it had apps.