I have one that I put on once in a great while for a different look:
Anyway, back to the topic at hand. It is made out of thin plastic, the mounting system is flimsy at best, and quality of construction seems to have taken a back seat to cheap pricing. It doesn't deflect much air, but the air it does deflect is taken off your chest area.
Mostly it's for looks, as I would have to be a good foot or two shorter for it to do any real wind blocking for me.
Of course, I'm 6'8, but that's really a moot point since I doubt it would block much air for a shorter person anyway.
I think it's more of a form over function thing, ya dig?