Went there to see the snow monkeys and relax for a few days. We stayed four days, which is way too many in the winter unless you plan on skiing or doing a winter sport out there because there is very little to do there in the winter other than the snow monkeys which is a half day affair, but that was our fault, not theirs.
The food was good and surprisingly varied, we were worried that staying four days would mean lots of repeats, but we only had a couple of repeat dishes, and given that each dish is so small (dinner is 10 or small things, breakfast is maybe 5), it wasn't an issue at all. We did their Japanese style food everyday so can't comment on the Western style option.
The rooms were fairly minimal but the onsens were quite enjoyable. Took some trial and error to get good at adjusting the temperature, but otherwise, was nice to jump in and just relax. The sleeping was a bit tougher. The pads they put down on the floor were quite firm (and I generally like firm mattresses) and you only get one very small pillow. Neither my fiancee nor I slept very well. I imagine it's fine if you're there for one or two nights, but given that we were there four nights, it was something we were both mindful of.