Just got my 49x850b set up last night. Based on limited use, I would say that there is little difference in picture quality with cable between a quality 1080p set and a 4k unit, certainly the way I have mine set up right now. On the other hand, fed a 4k signal (Netflix), there is a rather clear difference. As more 4K content is made available, the 4k sets should be quite a bit more worthwhile. I'm especially hopeful that when 4k Bluray arrives, the results will be nothing short of breathtaking.
By the way, I'm not ready yet to comment further on the set. I want to work with it more before weighing in.