I've used these stencils before. I would not recommend them. The icon stencils are cut out at the exact size of the icons, but when you're using a stencil you often have to make the stencil larger than the icon you're trying to draw. The result of tracing them often looks like you did a -2px stroke on whatever icon you're illustrating. Also for finer detailed icons, you often can't fit a pencil into the groove of the stencil, even with the provided pencil that comes with it. The lead of the pencil is thicker than the groove in the stencil. The edges of the stencil are often quite sharp as well, and will cut the tips off of felt pens (and create grooves in pencil lead which will cause it to break). Honestly the only thing I now use my stencil for is as a dull knife to open packages (you can easily cut paper, foam, cardboard, or tape with the sides of these things, they aren't sanded down at all).

With stencils use drafting pencils/pens that have a steel sleeve and are designed to be held at right angles to the paper.

Beware if you like sketching with pens only and use these stencils. Will screw up your pen tip.