I have played 25 days. I bought it at launch, and I try to play every day but sometimes I need to rest. The physical challenge is pretty high. I started at 21 difficulty but that was too high so I lowered it to 16 and that's been going well.
I would say that it is a nice sustainable practise. If you tried to do the same routine sans the game, it would be impossible to stay motivated.
The game goes through a warm up stretch and a cool down every session, which is an excellent routine that I often neglect when I'm in my own, so I do appreciate it.
For me, I already play Smash every day, so it is as simple as playing Ring Fit first.
I have seen good results. Tone in my upper body muscles, stamina in my legs, and strength and flexibility in my core.
As another plus, I have beat like 3 games since I started Ring Fit and I still haven't cleared Adventure mode.
I'm really glad I have Ring Fit to get me through the winter, when I would normally give up entirely on my fitness.