Reporting back on our Sunrise Seto experience. We booked our ride about 5 days in advance. By that time the car was more than half booked and we were only able to get two side by side which forced my wife to sleep on the nobi-nobi because we couldn't leave one kid alone. We had two upper berths against the bulkhead where my wife slept next to one of my kids. And then I had two lower berths on the other side of the car for myself and my other son. Since we would be getting hot & sweaty climbing the 1368 steps at Kompira-san the next day followed by a stay at an onsen, my kids and I didn't bother buying the shower card. My wife did however and she said she slept well.
The spots next to her didn't fill up leading me to believe that people changed their plans or just didn't show up. So if i were to do it over again, I'd probably check the seating situation one more time just before boarding to see if we could have been lined up as a family. But it wasn't a big deal. We could go back and forth as needed and we slept mostly considering that boarding time was around 10pm and arrival around 7am. We didn't even bother getting eki-bens since there was no need to eat on the train given that schedule. We had some mugicha and sembei for snacks though.
Note there are no places to put luggage aside from within your nobi-nobi space but a full-size suitcase could fit mostly out of the way across two spots.
As for the experience, it's somewhat like a capsule hotel except much less comfortable and less private. It feels like sleeping on thin carpeting over plywood (less pliable than tatami). You are given a thin sheet and blanket and no pillow. I used my air-travel blow-up pillow. I was able to sleep and certainly more comfortably than on an economy seat in an airplane but far from any normal standard of comfort. A backpacker sleeping pad would have been helpful. That said, the price was right and I'd probably do it again. And, most importantly, the wife (and kids) seemed to enjoy the experience.
