I used to have what was considered a really good upscaling DVD player back when I was in my old house with a 104" screen. I thought that it was a great image. Then I got into HD-DVD and eventually a blu-ray player. They were noticeably better than the upscaled DVD image.
I assume that the UHD will be the same over "upscaled" blu-ray.
I think that the biggest thing is that if you never see UHD content, then you will never know what you are missing, but as soon as you do, that will be all that you want if you want the best possible image.
