I had the Lumia 520 as a review unit, for both Windows Phone 8 and the device, for about a week and a half.
I found the phone itself to be great, apart from the rough plastic back, it didn't feel like a £100 ($150) phone. It was smooth in operation, this is partly due to how WP8 handles transitions and animations, and battery life was sufficient.
For the price, and if you aren't coming from either iOS or Android it will be a brilliant phone. If you move from the other platforms it will have a steep learning curve and a lot of apps just aren't their, either in direct form or as a substitute.
I've written a review
here