I've done it, it's quite fun. You get all kinds of cars participating, even people with minivans. One guy in his minivan was doing handbrake turns the whole day and he was having lots of fun.
Since they run all the cars on the same course during the practice, with my AWD with good winter tires, I found that I had to go really slow on the straights to make sure I didn't catch up to the next guy, so I could get some speed around the corners. Many people go to the school with all-season tires.
If you've done Solo II before, you might not learn anything new, the course covers mostly the basics. But for most people, I think this is a must-do.
If you've done this course, then go out to one of these and put your newfound skills to use.
http://www.mco.org/app/winter_slushnslide.asp