I have the BMW cover for mine. http://accessories.bmwusa.com/ItemView.aspx?modelId=240&categoryId=19&productCategoryId=&menuId=0&subItemId=1&productId=124
Used it in the winters in Wisconsin so it was covered in ice and snow and held up fine and protected the car extremely well. Now I use it when I leave my car at the airport for extended periods of time.
The key thing to these car covers is that they breath. So if it rains or snows and moisture get between the cover and the vehicle, it can escape and save that lovely paint job! Trapped moisture is BAD!
I bought an Autopyjama, quite expensive but beautifully made. Mine's an indoor cover for winter storage, but they do outdoor ones too. I searched forums before buying and they were highly rated. I'm happy with mine.
I have the Bavarian Autosport Ultimate Car Cover. I had it on my Z4 all winter through many snow storms and when I took off the cover it was as clean as the day I put it away. It is a very thin material which is nice and I accidentally had it rip on my license plate frame and they sent me a new one for free. Other than it's price there is nothing negative I can say about it and it is easily the best cover for outdoors that I have ever used.
It does fit very well and you can run a cable which is included to really secure the cover for high winds.
Thanks. I was up on BA earlier and I kinda like it...
I am leaving town on Tuesday, and I want to get a cover that is easy, (translate: brain dead easy) so my wife can put it on the car..without scratching it....
I use the BMW cover also. Fit is excellent but it does come off the front and rear in a high wind. I'm going to modify it to secure it under the bumpers which will stop the lifting problem.