The hot spring at Futaba Onsen is brownish with a light iron smell that stains the areas and leaves a calcium crust near it much like Hamada or Ogo onsen in the area. While the water is good, it's in no way the main focus of this sento. I've always thought the normal bath and the outdoor bath were the main features.
The main normal bath is always piping hot. The design is such that there are many little corners to sit or stretch out in. Sadly the two strong jets are inconveniently placed making them useless for me. They are good for feet or lower back, but not much for the upper back here.
There is a large shallow outdoor bath here. It takes up about 40% of the entire bathing area. It's decorated by a waterfall rock feature with real plants. The view is straight up into the sky. In the summer coming around dusk and watching the sky change colors makes sitting around the outdoor bath an absolute treat. You can pass out there for a long time, they even have a sign warning people about staying in the bath for too long. Besides the rotenburo there are two other baths using the hot spring water. A normal one in the back and a bubbling one is placed in a conspicuous spot right in front of the door to the changing room that might deter some bathers.
I love the great big ゆ sign in the parking lot along with the hot spring waterfall out by the sidewalk. In side the changing room and small lobby is basic but welcoming. The family that runs the sento seem friendly enough. I've seen people I know and my students here on occasion.

**UPDATE** It seems that everyone’s quit doing this.
The regulars like to cover the drain for the outdoor bath making it overflow, you'll notice a small CCTV camera pointed in that area. I like it when they do it (if you go when they do it you'll understand) and I never take the bucket that's covering the drain off, but I'd never put it there myself.

**UPDATE** Futaba is also special for me because it was the first sento I went to, and for a long time the only one I knew of. As an exchange student I would go there at least once a week, but typically more often. It's the closest sento to my house and I still enjoy coming here.