Hi all,
I think it works great and I wouldn't want to do without is anymore.
They will work. Mostly. The listed "filter" contains anion filter resin. Given the asking price I'd question the quality. Your local water filter shop or online water filter supplier can provide standard cartridges for standard water filter modules. The resin modules are re-chargeable and re-fillable as needed.
There is nothing wrong with these products at all, they work and will reduce your NO3 levels, but they are unnecessary in the planted tank, the plants do it for you very effectively and cheaply.
If any-one is interested in how these canisters work, they active bit is an "anion exchange resin" polymer as dbosman says. In this case it swaps the NO3- ion for a chloride ion (Cl-) in the incoming water. The unit is re-charged with a strong salt (NaCl or KCl) solution when the exchange sites are exhausted, in this case replacing the NO3- ions with Cl- ions in the reverse of the active process. We used to use a lot of Amberlite resins in our waste water work, but again they were a lot more expensive than the posted link.
The only other way to get rid of nitrates (if you don't have a nitrate free source of water for water changes) is to reduce them to N2 gas, this process is bacterial and occurs in anaerobic (oxygen free) conditions. It has had a vogue in Marine circles with people using de-nitrifying coils, deep sand beds and Jaubert plenums, but it is a difficult process to control and has mainly fallen out of fashion.
If any-one is interested in a bit more reading, I'd recommend these pages from the "Skeptical Aquarist" <
http://www.skepticalaquarist.com/docs/nutrient/nitcyc.shtml>
cheers Darrel