I used one once. Unfortunately for me, I had just shaved that morning. I did not realize what a bad idea that would be. The salt got into all the tiny exposed hair follicles and it burned badly.
The first time I used one I just panicked for the first 20 minutes. I had to really talk myself through it and convince myself that I was safe. It got easier after that. You are alone with your thoughts, but you can focus on the music or the sound of the water as you move it around with your hands. The most uncomfortable feeling I get now is boredom.
The one I use has a door, so you enter it like a shower. It’s more like a room than a tank. That helps a lot. I’m not sure I’d be able to do it if it was like a tanning bed or a coffin.