I've had a 520 for almost 2 years now. Loved it at first, feel pretty fed up with it now. The earlier WP8 version was faster and the People hub worked really well, however the WP8.1 update pretty much killed it, meaning that I need to use the 3rd party apps much more. The Facebook and FB Messenger apps are ridiculously slow, the Maps navigation is unreliable, the browser is limited (really MS, STILL no forward button?).
I'm due a new phone in a couple of months. No doubt it'll be an Android, and I can't wait.