If my car sat out I'd definately get one. But I would leave it on all winter-storing it. I don't know how that would hurt your paint. I have bought a couple covers from this company: http://www.calcarcover.com/cover_main.aspx
If I decide to store my new Si I will be purchasing from them. :thumb:
Personally, I have a car cover that is used for a vehicle that is in storage for the winter (garaged too). I don't think I would use a car cover for a daily since there is just too much potential for scratches and such.