I usually run with vsync on, no point in generating the extra heat, and using extra power to produce frames I'm not going to see due to my display's limitations. I generally only turn it off in highly demanding titles where I can't get a solid 60FPS with my current hardware, ie. Crysis, or when vsync is either causing performance problems, or the option is horribly broken ie. Dead Space with vsync capping the framerate at 30FPS, and introducing severe mouse lag.