I just made the switch last week from a S4. Overall in really liking it, I don't think I'll ever go back to Android. I picked up the Nokia Lumia 521, the WP just works and it's very fluid. I really haven't found anything I dislike about it. Although there is no support for 3rd party keyboard apps or keyboard vibration feedback. The YouTube app sucks but one called Toib is in the making at the moment. I like the built in Internet sharing feature which is a WiFi tether thing. Also plays games good, I got Halo: Spartan Assault, Asphault 7 and 5 and Six Guns + Warspear Online on it atm. Asphault 7 and 8 even support my Moga controller. And the Nokia RadioMix app which is like Pandora is AWESOME.
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