Sorry to double post, and a long one at that, but I've been giving this thing some more thought (more than it probably deserves).
Anywho, I suspect I know how this thing "works". Firstly, it is probably lined with a nice absorbant material that will, in fact, make your water nice and crystal clear. (See, for example, the Algone pouches). And you remark, "wow, that water sure is clean and clear, I guess it really does work!" So, you start to skip 3 out of every 4 water changes. And, guess what, no problems occur. For 6 months or so. Then, the fish get a disease or a few pass on or maybe even as simple as they start to lose color and act listless or maybe a bit more aggressive. But, more than likely, you have bought a new peice of decoration, a new piece of equipment (that bigger filter you've been eyeing), feeding a new kind of food, you used medications to treat for a 'small outbreak' of some disease, or the biggest one of all -- you added some new fish. So, this new disease or symptom you blame on the new equipment, decoration, or inhabitant, and you continue to skip the suggested water changes. You curse your woeful LFS for selling you weakened or poor or sick fish.
You may not come to realize that it was this useless object that is causing all the problems -- well, in a roundabout way since you have not been changing your water as often as you should and so your water quality has gone to hell. Your water has no more buffering. All the tiny parts per million and billion of paint, carpet fumes, cooking fumes, etc. that buildup, albeit slowly, has not been removed as quickly as you might have before. You know not to top off but via evaporation, which as we all know is only pure water, these tiny amount of contaminants have been left behind and thereby have been concentrated. That, and not all the fish waste comes out as nitrates, some will get eaten by other bacteria but some stays in the tank until you gravel vac and water change.
So, in synopsis, you have been doing your job as a good fishkeeper and your tank up till now is doing great. By adding this magical device that supposedly allows you to skip 75% of your water changes, you begin to do so. But, and here's the kicker, it takes a fair amount of time for your well-maintained tank to become a tank in poor quality -- maybe as much as a year.
I don't know if anybody remembers the ceramic washing disk scam that ran ads in the newspapers in the 60's and 70's. They were these inert ceramic disks that you supposedly could toss into the washer and use them forever instead of buying liquid washing soap or powder. And, guess what, they worked for the first 3-4 times you washed you clothes with them. Then, they seemed to lose their potentcy. Well, what really happened is that the disks were completely inert... did not do a thing. But, our clothes have enough dissolved soap in them that you can wash your clothes without soap a few times (e.g. you can always tell who washes their clothes with powder if you turn on a black light), and the soap that comes back out of your clothes does a sufficient job cleaning them. But, you eventually take all the soap out, and the 'disks' stop working. The company for a while got away by telling people that their water was different, or that a chipped or cracked disk didnt work any more.
I think that this waterchanging saver is the same thing. You wont blame the device because they are hoping that problems will not occur for some time, and that there are a great number of things that you may blame between begining use of this object and the problems. In the best case scenario, I see this thing having no effect and you've wasted your money, but I can also see all the long-term problems listed above, and maybe several more I have not thought of.
Again, there are plenty of people willing to sell you something by telling you what you want to hear.