I just used it and this is what I did.
First I let the buttercream CRUST. Then I used a combination method of smoothing the icing with a papertowel. I rubbed the paper towel with a fondant smoother, against the cake. Personally, I did not like the imprint of the paper towel, so I switched to wax paper and then to parchment paper, which left no imprint.
When all the bumps and spatula marks were pressed out, I then proceeded to use the sharp edge smoother. My top edges and corners were RAZOR sharp. And I never used the edger directly against the buttercream, because it would always stick. I kept the parchment in between my smoother and the cake, folding over the top edge or side corner.
If you try to use the edger with fondant, I would apply the fondant and use the edger immediately after fitting the fondant to your cake. The fondant is still soft and pliable. If you wait, the fondant with get too firm to make much of a difference.
Good luck everyone. I will post a picture of my small sample cake that I am working on now for everyone to see.
I am pleased.