I have always passed on these for fear of strop contamination from laying the strop down. Think about it...any time you use one side, the opposite side is probably resting on your bench...then you rotate the strop to the next grit, set the strop down on a new side and voila, you have cross contamination of various grits on the strop.
Perhaps this concern is ill founded, but that is the reason I have not tried one...