Why not activate it and see it for yourself? It takes like 10 seconds to try it out jesus man it would have been quicker to just boot up a game, try it out, and see for yourself rather than post here.
Vsync smooths out the framerate if your FPS is higher than your monitor's refresh rate. It introduces mouse lag, which is dumb in fast paced games, but it negates screen tearing, which is dependent on the game as to how bad it is. But it also reduces framerate by a small margin.
It's personal preference, some people don't mind the mouse lag, others do. Other people don't mind screen tearing while others do as well. Go try it out, this isn't a "should I do it" question, it's personal preference.