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C4 guys raved about them, so...
I bought one for our C5, did a whole afternoon of comparison tests. Worthless.
Haven't bothered to try the C6 version yet.
The good news. The claims that it reduces turbulence are absolutely true. You get a nice clean flow of air through the car, rather than a turbulent swirling flow. It does work.
The bad news. Even though it seemed to fit when I closed my hatch, after driving for a while it would put forward pressure on the C6 trunk latch system, and actually forced the latch mechanism forward past its' mechanical stop. (Not good - had to bend the mechanism back without breaking it). Had this happen twice; spoke with Jerry Thorlton the inventor/owner of Ventmaster who advised me he had not had this feedback before.
So my opinion is. . . if you can find a product that actually holds the trunk up without destroying the latch then it would be a good thing. And if anyone knows of such a product (not Ventmaster) I would like to know.