I do not have one, but over the years, I have read that people that have the jacket like it. It is really meant for those who have a control unit (a fan that regulates air flow and thus the temp), but it will work without the control unit. People do stick the probes through the top vent, but as you might surmise, it will be a pain.
Why do you want a jacket? I would suggest that there are other alternatives that work as well or better (if you define better as easier) and most are cheaper. The WSM will run very steady if you can get it out of the wind - even in the coldest temps. One thing I've seen is using a welders blanket and some clamps. They just wrap the blanket around the unit and it works very well. Also easier to handle the probes because it's not a jacket. No sure how that would work in the rain but in a light snow, I think it would work fine.
If you're looking for something to get out rain and wind, many here have built a shelter. Depending on your skill set and source of materials, this could range from very cheap to more expensive than the jacket. The jacket does have the advantage of being water resistant.
I'm lucky enough to have a covered porch so rain is not an issue. If it's windy, I just tuck the WSM in the corner and block the wind with my Genesis. Even before the porch was built, I just used my WSM on the side of the house that best blocked the wind. The worse day I ever had was when it got windy and we had a downpour. The hard rain sucked the heat right out of the WSM. A light rain or snow never impacted a cook, but that day was a bad day to be outside. Personally, if I didn't have my porch, I'd build a little shelter. Look in the mods section of the forum and there are a lot of ideas. I've always liked the ones that were made to be easily broken down for easy storage.