Cwah tried one of those on his Brompton. Unfortunately, it didn't perform like their claims. Have a look to see if you can find his thread if you want a good laugh.
On a slightly different tack: I was in Aldis today, and I managed to snag a pair of waterproof spats (for my feet) for £4.99 - reduced from £6.99. Handy on the motorbike too because I hate wearing boots.