Yes, they do work. It is not really that different than a Nature2 cartridge. Yes, it is bigger and more expensive - and they claim it can go six months. But two of them cost just about the same as three N2 - and I personally would rather have the extra water change.
Water is the cheapest chemical you can add to your spa, and it has the biggest effect on clarity, smell, balance and 'feel.' I have plenty of D1 owners who come in to buy their N2 cartridges every four months...