Vsynch uses a lot of system resources, furthermore it uses ratios in relations to your screen refresh. Typically most LCDs run at 60hz, some support 75hz (but are often only running at 60hz). Why this is important is vsynch clamps your fps to a ratio of your refresh. If you can reliably get 60fps it will clamp it to that, but if it ever drops down below that (even by 1 fps) it drops down to 30fps (1:2 or 1/2 of your refresh rate), and if it drops below that it will drop to 15fps (1:4 or 1/4 of your refresh rate).
The reason this happens is because vsynch is designed to eliminate any screen tearing effect. This effect is more predominant on LCD screens, and less so on CRTs. Vsynch also adds measurable input lag compared to having it disabled.
Unless you have a very good reason to run vsynch it's typically not a good idea to run. It chews up far too many resources for a marginal benefit. If you want to see a bigger benefit to your gaming (or video) experience, get a 120hz LCD monitor and run it at 120hz (I do not mean to use it for 3d purposes, I mean to use it for 2d purposes). Getting a good 120hz monitor will be far smoother than running vsynch and not rely on such a hog of resources. Such a monitor looks best however if you can reliably maintain 120fps or more.
All in all, I recommend you do not use vsynch.