I have the 3M clear bra on my '07 Boxster and it looks & works great. It saved me the other day on the way to L.A. I had a construction truck ahead of me with dirt flying off it. Before I could change lanes I heard the sound of ting, ting, ting as the debris was hitting the front of my car. Needless to say not one mark when I checked my car.
From 3+ feet away you cannot see it. I think the advantages of the clear bras over the standard bras with the Porsche are pretty much the same as other cars.(ex. appearance, mold, scuffing, etc.)
http://i178.photobucket.com/albums/w274/r9i8c7k/DSCN0986.jpg