I have the smoker jacket made by BBQ guru. At about $100 for the 18.5", it's not cheap, but it does work well!
http://http://store.thebbqguru.com/weborderentry/18.5%22%20%20WSM%20Smoker%20Jacket
I used it in. BBQ compitition and we had heavy rain and wind up to 40 mph at 50 deg F. With this jacket on, my weber temps remained stable through the night. I have also used it on days in the low 30's with no problem. All though I haven't used it at temperatures and weather that would be considered extreme, I Have yet to find its limits.
I have used mine in very low temps (0F and below). The insulation qualities are very good. You will get condensation that will freeze on the ground around the WSM but that's hardly the jacket's fault