Looks excellent to me. I've been using it on my Shield TV Pro for several years and it's amazing how well it can work on the big screen.
I think mileage is going to really depend upon the source and what you're viewing it on. A decent 720p source is going to look a lot better on a large 4k monitor. On a smaller 1080p screen the difference may be negligible.
I can totally understand how someone on a regular 24" 1080p lcd thinks the whole thing is a waste of time, and how someone on a 45" 4K oled thinks it's the greatest tech in the world.
Most people will be somewhere in the middle, where it gives a nice little bump in quality, but nothing to buy a new video card over.