They do work, as well as my bar inner weights that I run all the time to reduce my bar vibration.
http://www.xt660.com/showthread.php?t=12164
One other thing I did was to place plastic washers between the bar mounts & the top triple clamp, this helps reduce the vibration being passed to the bar mounts & bars. I also run one wrap of cloth tape rapped around the bars between the bars & bar mounts, this separation reduces the vibration to the bars, I have never had the bars move in the mounts at all with the cloth tape, if you use insulation tape it will move.
My XT's bars are so smooth with all these mods.