We stayed overnight at Aoni Onsen during the middle of the winter in January. During the winter, you have to park at a visitor's center off the main highway and are taken up to the onsen in a bus due to the deep snow and narrow road. (Some people opt to cross-country ski into the resort, but not us!) The drive to the onsen is spectacular in itself, with huge mountains of snow on all the surrounding hillsides. Once at the onsen we were greated warmly and shown our room, which was in an out-building from the main resort and had a thatched roof! The staff (no english here) was very helpful and, as mentioned, friendly and the baths were amazing. A must, especially in winter, is the outdoor bath, which is mixed-gender. It's really amazing sitting in the onsen with 2 feet of snow on the ground around you. Dinner was probably the best I've had at any of the onsen resorts I've stayed at since I've lived in Japan. I will definitely come back here next winter!