Just received the Mozo Zebra Wood replacement rear cover for my Lumia 950 and I have to say it looks and feels fantastic. Without adding a millimetre of thickness to the size of the phone. With the rather nice Mozo leather flip case and original polycarbonate rear cover I've a good selection of outfits for my phone. The Zebra Wood is definitely the nicest so far.
Tied with the user replaceable battery I'd say a replacement back cover is the best argument against unibody smartphones there is. Especially when the quality is this high.