To add to the above, I finally did try one on my last trip to Japan, when I was traveling solo, and when the next cheapest option that I could find in the city was $250+ more expensive.

It was definitely tolerable to save that kind of scratch, but I would absolutely never go out of my way, or encourage anyone else to go out of their way, to stay at one. I actually liken it more to being like a youth hostel with absolutely huge rooms, and some curtains around your bunk.

Anyway, I left my wallet and passport in my locker.