Yes, I have. And I can confidently say that Mac OS X, even today after a decade of evolution and sweeping changes, is clearly a direct descendant of NeXTStep and has little in common with System 7. I'll also reiterate what others said -- there is everything wrong with Gates' statement.

I do iOS development now, and I spent the 90s doing NeXTSTEP/OpenStep. I still own two NeXT machines.

The main differences now are 1) cheaper, faster hardware, 2) menubar at the top instead of a floating vertical menu, 3) I keep the Dock on the left instead of on the right side of the screen, 4) fancier development tools.