Gysnc is defiantly good, you get no screen tear, no input lag, a clear picture with no blur even when turning the camera as fast as possible. Gameplay is much smoother and 40fps feels more like 60fps on a Gsync monitor.
You don't realize just how much better it is until you have seen it for yourself. I tried playing a couple of rounds of BF4 on my non Gysnc monitor the other day and I noticed tearing that I never noticed before, the game didn't feel as smooth and the picture wasn't as clear. It is one of those things you have to try for yourself though, in my opinion Gsync is good, not sure if it is worth the price premium you pay for it, but it is good.