Poison Cleansing Totem was highly useful against any creatures which poison consistently (such as an on-hit poison or an AoE one). It was slightly useful against creatures which have a poison spell, as you would not get poisoned enough to make this more useful than a specific poison-removing spell. It was completely useless against mobs which do not have a poison ability.
The more poison your team is fighting, the more useful this totem would be. However, it was possible to get poisoned so much that the totem couldn't keep up. The totem would stack if different shaman drop them, since each one would pulse and remove poisons separately.
Patch 3.1.0(2009-04-14): Poison Cleansing Totem and Disease Cleansing Totem have been merged with into "Cleansing Totem." Cleansing Totem pulses every 3 sec, down from 5.