I have done it a couple of times and I can tell you that it is time consuming. If you use a buttercream recipe with crisco in it, you will have better results than with a softer buttercream made of 1/2 butter 1/2 Crisco.
I do like this technique, but instead I have decided to buy the large putty knife and run it around the sides and across the top of the icing while it is still wet. This will get it fairly smooth for you and then once it crusts you can take the Viva papertowel and smooth any lines you may have left.
Also, the upside down method really only works for smaller cakes since you have to be able to flip it over after it's iced and the larger cakes are heavier and more unsteady.
Hope this helps!
Brandi
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