We went to Cassi onsen in the middle of winter. It was a nice winter retreat. We came from Nasu along route 290 and were very, very glad of 4 wheel drive and snow tires. I think overall its much smarter to drive in via route 289 in winter.
We stayed at Gohoushou. Its a good, though not remarkable onsen.
Food was good.
Onsen was better than good and beautifully lit at night.