Keep in mind I only played with it a little bit last night, so my overall impressions are still being formed I guess. I only tested BF4 and my old monitor is 1080p @ 120hz TN, so I am used to high refresh.
75fps looked and felt really bad, regardless of Gsync. This surprised me and will cause me to investigate and play around with it more. I don't remember 75fps looking that crappy, but then again, the frame rate was very unstable and performance kind of sucked because my 2GB cards were choking to death with my old 1080p settings (Ultra + 4xAA). So i'll check it out later.
At higher FPS, Gsync added a kind of "completeness" to the image. It certainly felt nice to have no tearing and the image was just solid and uniform. I like that, but to be honest I didn't see a big difference in motion clarity over a standard 120hz screen without Gsync. Again, with more usage my opinion may change.
I have only a couple hours experience with it and I did notice a smoother feel to the game once the FPS was higher, but not having Gsync wouldn't be a huge deal IMO, mostly because if your FPS is low to begin with, the experience will suck anyways. I think a gamer is much better off with simply having high FPS and 120+hz. Gsync is the icing on that cake, but its just the icing. Its not the whole cake.
Now, colors and image stability are far better on this thing than my old TN. That much is perfectly obvious. So, adding all this up together makes for a really nice upgrade from where I was at. I am slightly bothered by a yellow tint on the lower half of the screen, but I can live with it.