Well, I haven't tried the Slipstreamz but I've read about them in a few posts since searching on "wind noise" and the critical performance requirement seems to be that the equipment interupting the flow of air past your face needs to be extremely close to your skin or touching it (try the finger test from the Slipsteamz site, where you put your index finger up along your face in front of your ear while riding).
Orientation of the device is important too, and I think the more perpendicular the device is to the flow, the better it will end the noise. I like a DIY challenge so I made a couple prototypes, the first out of a water toy laying around the garage (a funoodle, closed cell foam stick), and then more recently out of some softer foam rubber plumbing insulation and an old rag. Now I just need to find someone who can sew on a machine to whip me up a few sets that will look good and be durable thorugh the wash from time to time.
I have an advantage on some of you in this aspect of wind noise reduction because I'm riding recumbent and that puts my head up at a better angle to effectively end the noise. I find riding with these (pics below) I can now hear the road noise from my tires, along with all sorts of other new noises. If you ride with music of any kind, this would let you turn the volume way down while hearing everything a lot more clearly. The tradeoff is that you have to be willing to ride with something touching your cheeks, hence the more compliant foam with the cloth wrapper for me. When you try the finger test, experiment with pulling the finger away to judge the effect of even a slight air gap on how any system like this would perform for you with your head and helmet in their normal riding position.
