At one point the showers were reserved for First and BusinessFirst passengers, and one had to produce a boarding pass, not sure if that is still the case.

I've used those showers at EWR. It works like this:
There are multiple private shower rooms along a corridor off to the side of the lobby. Each has a shower, toilet, sink and dressing area is a decent size. Access is controlled by an attendant who has a desk at the end of the corridor. There is sometimes a wait and you put your name on the list. Towels and products (shower gel, shampoo) are provided. The facilities are pretty well maintained and clean but as with any shared facilities there's always the possibility of athlete's foot, but no worse than any gym I've ever seen. There is no charge, though most people offer a few bucks gratuity to the attendant.