Yes! I have turned that on many times and I always end up watching flash videos again. I wonder if it had to do with my session cookie expiring. Does anyone know how they toggle this experiment on/off for different users?
I'm in the beta, I've never had to rejoin. Possibly because I'm logged into my Google account, which is linked to my Youtube account. Note that some Youtube videos are still delivered as Flash, I believe is whenever adverts are shown.