Been using an Ativ S for about a month coming from iphone 4S. Overall a great device. Most points of OP are right.
Sound quality is a big improvement over the iphone and the screen rocks. Love the 4.8" screen and the responsiveness although sizewise it is for me at the limit for a phone.
Also the camera is better than i expected.
I do not like the back though. I never use cases and the plastic scratches. Back cover easily, sides less so. The phone will look brilliant and fresh up front but the back will look used and "dirty" very soon.
Price tag for the spec of the device is amazing.
Love wp looks and interface and want to see it develop.