Pulled my spare 520 out of the drawer, updated it to 10536.1004 and did a hard reset. Unusable at first, while it downloaded 40 app updates (to be expected). In playing around with it, it seems to be slower than it was with 8.1. I'd hold off for now. Still seems a bit buggy at times, and I've even seen where the phone wouldn't want to rotate (reboot fixed it though), or some things won't let me swipe to type. I also much prefer HERE Drive+ to Maps for navigation. I was trying to use this as a test bed to see if I should put 10 on my 920, and I'm thinking I'm going to wait (or simply not, and just upgrade to a 640 or 950, but after things are fixed). Remember, the 520 was the $100 smartphone from three years ago.