Well I've had My Injen HydroShield on for a little while now.
Performance wise... It feels to strangle revs at the top end a little, but I don't really go mad in winter. I might VTEC it from 4500 to 6000, but that's about it.
As for dong what it says... I've driven in some appaling rain, spray and real wet roads, and so far so good. It's not missed a beat once. Definately worth the £20 delivered. 8)
AEM owners, go get one!