It's incredibly ticklish at first -- enough to make your squirm. The fish are supposed to nibble away your dead skin and leave your feet nice and smooth. Many are branded Dr. Fish. The fish species is garra rufa, which are small carp that live in warm water, have no teeth, and like dead foot skin.
I got this done in Siem Riep, Cambodia but apparently it started in Turkey, spread to Asia and is now quite controversial in the United States. The state cosmotology boards are going nuts. For example, shared foot tanks have been deemed swimming pools in some places and shut down.
I paid $3 for 20 minutes, so it gets less the further away you get from the main thoroughfares. Some places even give you free drinks.