Here is a long thread discussing Nitrate Reductors from AquaMedic which are very similar to Aquaripure filters (which are discussed in the thread as well).
Nitrate Reduction (56k Warning) - Perth Cichlid Society Forum
Aquaripure has its own site if you want to read some more.
Aquaripure Nitrate Filter
You can contact the owner and creator directly if you have specific questions as he has responded to all my questions quickly.
The Aquaripure filters need to be feed manually (injected cheap Vodka) on a regular basis. The AquaMedic filters use a special bioball food that breaks down by itself and only needs to be changed yearly once the filter is established.
They both work the same way internally (grow same oxygen starved bacteria) and are slow drip filters (2-5 drips per second). Because of the lack of oxygen (slow water pass thru rate) they are forced to feed on the Nitrate in the water and convert it to harmless Nitrogen.
The consenus seems to be that they extend out the time between water changes (double or triple) eliminating some tank maintenance. You still need to do some water changes though to pickup important minerals in the new water.
They may become more common in the future if water prices rise or water is rationed. The thread above is from Australia where severe water rationing is in place and people are looking to do less tank water changes.
Here is also link to the AquaMedic Nitrate Reductors
Nitrate Reductor - Aqua Medic