Think about the situation for a second. How much time does one spend in a public washroom, health club locker room or go to the water fountain over and over? Studies done on indoor advertising have proven that it can be from 30 seconds to as much as 4 minutes. Your audience is truly captive. The ad is in full-color and set only 1-4 feet from the viewers eyes.
There is absolutely nothing else to look at, so people will read the ads for as long as they are there.

If it really works, what companies use it?

"If you went to the restroom three times, you'd read it again and again. Not only can indoor billboards reach a very discerning customer, but they've got that customer's undivided attention."
-Inc