The liver (Kimo) usually is used in a clear soup (sumashi). A light broth with little in it other than the kimo that accompanies the main eel dish. I've had it countless times but the liver itself is pretty tasteless and kind of chewy like rubber. Really nothing to get excited about. Particularly when the eel itself is so strongly flavored. I suspect it just goes back to the days when as much of any fish was used.