I'm been using a Windows Phone for 1 month (from iOS). Surprisingly, the pure OS part is not bad at all (even with some perks like BT aptX, default maps app with offline mode). Store is missing a bit of stuff (mostly first class games) but I was able to find a replacement for everything else.

And one thing huge (IMO): the performance/price is pretty amazing with the low/mid level entry Lumia.

(I have the Lumia 735, iPhone 5 before).