I put the breath deflector on my shoei and I hated it. the problem was that the air that leaked through the shield was deflected right into my eyes, causing my eyes to constantly tear up and become irritated. I like the way it looked, because it almost looks like you're wearing a fighter pilot mask, but it caused too much discomfort.
the reason most people use them is because of shield fogging. If that's the case, simply take you lens off, clean it with warm water and toothpaste, then apply some No-Fog spray to the inside of the lens, and buff it with a tshirt. you can usually buy no-fog spray anywhere, Sports Authority sells Speedo No-Fog in the diving section for snorkle masks. It works awesome on helmet lenses. Any sports/diving will sell no fog. Spray it on, and buff it good with a tshirt so you dont get a smearing effect.
I think I rambled too much
Mike