I have a Noah cover for my Mach.. Keeps the dust off the car pretty good. However, when it rains the cover just absorbs the rain. So the cover is like a wet towel somewhat that lays on the car. It doesn't repel water at all from the car. You'd think it would with a name like "Noah"...
I had the weathersheild cover (got it from autoanything.com), but it didn't do the job very well. I returned it and got the Noah. Much better cover, it keeps more water and dust off the car than the more expensive weathershield. BTW, my car sits outside all year.