I would love to put a cover on my X350 for it's winter hibernation but I'm a bit unsure if it's a good idea or not. I bought a cover a couple of years ago for about £200 and the fit was terrible. It was just a generic cover for a large car, even though the ad said it was specifically for my car. The instructions said that it should be removed every week to check for paint damage and it wasn't completely waterproof. In fairness the company gave me a refund, minus postage of course. Unfortunately I can't remember the name of the company but if I come across the recipt, I'll report back.
My XK8 has a Dupont Tyvec cover and the fit is brilliant but I only use it as an indoor cover, not sure if it would be suitable for outdoor