My personal opinion is it's a nice visual indicator if using during cycling and testing regularly as well, but in a fully cycled system, it's totally useless. If your system is cycled, there should be no ammonia. So to me it would be a waste of money.
55G Filstar XP3 - 16" Fire Eel
75G Medium Planted - Filstar XP3 Low Light - Established Feb 2006