It's a shame it isn't. i have the S27A750D color output is better then ips monitors i have tried. 120hz i quite possible the single best upgrade i have ever done for pc.
It's not even about the frame rate even. Everything just feel very smooth and responsive and i can't explain why but i never really have vsync issue's anymore.
Are you actually running games at 120fps because if not then you probably just upgraded from a monitor with bad input lag or response time. 60fps on 120hz won't make a difference unless you have Gsync.