I did yoga during the winters for 3 years ... the first two years, I had a really good instructor. The 3rd year the instructor was OK, but I didn't feel I had gotten as much out of it, and I got busy with other things, so I stopped going.
I keep thinking I might start taking yoga classes again, and have been eyeing a yoga/pilates type of class that is held on Mondays ... maybe ...
But I might wait till winter before I start. Given a choice, I'd rather be outside cycling or walking, especially when the weather is nice.