I've only stayed in their "mini suite," but that one is on a higher floor overlooking the lake. Although it's called "mini", it's plenty good. In terms of the size of the onsen tub and the room, it's pretty much on par with just about any other private onsen rooms I've stayed in anywhere else. I'm not sure about their policy on solo guests.

In terms of the quality of onsen, I'd say it's average. Not quite as good as Hirayu or Kusatsu, but certainly better than Isawa or Kinugawa. I've only stayed at Shinyu in the winter time, which was the perfect time from the onsen standpoint. The cool air chills the en-suite rotenburo enough that I can be running the gensen (onsen) faucet at full-go continuously and the bath is kept at perfect bathing temperature. If it weren't cold outside, then I may have needed to cool down the bath with tap water, thus diluting the onsen content.