After much reluctance, I had to replace my 1520 because it just bricked on me one day. After much contemplation I decided to go with the 640XL because it was already compatible with Windows 10 mobile. I've been using it as my daily driver for about two months now, and I couldn't be more impressed with the build and more excited for what's to come. Despite the 1520 being of superior specs, the 640XL has handled WP10 so much better. Of course there are glitches here and there, but overall I am thoroughly sold on the direction of WP10 and would recommend the fast ring to anyone who has a 640XL. Hope this helps.