I've used it a couple of times. It's a bag of macro-porous synthetic polymer that's supposed to reduce nitrate and ammonia. It starts off as an off-white colour, then progressively darkens - which apparently means it loses its effectiveness.
The instructions say you can regenerate it with bleach - but I was too scared to muck around with bleach on anything that would go in my tanks. So I threw it out after a single use.