G-Sync is great if you want to run games in the 40 to 60FPS range
Say you want nearly maxed out visuals on an older GPU. It lets you keep a GPU for longer.
If you have a GPU that drives 60FPS with vertical sync on i.e. you have enough power never to dip below 60FPS then it is a waste of time IMO
Depends a bit on what FPS you want to game at...and if you're one of these people who think 60FPS is visibly slow to you then G-Sync is going to help in smooth gameplay at whatever variable rate you achieve over this...
It's a good thing to have...but not really worth it over 60FPS for most people...though people can be convinced of anything it seems;)