I've owned one for a couple months now, but the hard top has been on the whole time so I haven't used it yet.
Using common sense, I would think that when it rains, there is usually wind. When there is wind, water can get pushed up around anything, including under a cockpit cover. I would use it in fair weather to keep sun, leaves, and bird doo-doo out of the car.
Rodd
Powell, Ohio, USA
1966 230SL, Euro, Auto, Leather, both tops
1994 E420