I use the Carbo Plus on my 46 gal. fw aquarium. It seems to work reasonably well. I use a mix of 3/4 r.o. water and 1/4 tap water, and keep mostly tetras and a couple of other fish (like my genius E nose and Hasbrosus Corys) that like soft acidic water. The tank is well planted, with lots of various crypts, dwarf and standard sag, Amazon Swords, and others. The ph holds at about 6.7 to 6.8, with a carbonate hardness of around 3 or very slightly higher. This gives me moderate but adequate CO2 levels in relation to the lighing, a 96 watt c.f., and a 30 watt standard flourescent. Plants grow nicely, though not explosively, and things seem very stable a year after I first set the tank up. Initial hair algae problems have largely disappeared with strong plant growth and fine-tuning of light duration and other parameters. I fertilize the system very lightly and add the minimum in replacement trace elements. For me, the fish are the first priority, and the aquarium's chemistry is focused on their needs. It is a fish-with plants rather than plants-with fish aquarium.
The Carbo Plus is on a timer with the 96 watt cf, with both on for 101/2 hours each day. The 30 watt is on for a couple of hours longer, as a dawn/dusk light. I just installed the third carbon block. They seem to last about 5 to 6 months with my set up and water chemistry.
The best thing about the system is its size and simplicity. It is more expensive than other methods I have looked at, but I would not use the compressed gas system unless I were running multiple or much larger tanks. For this one medium sized tank set up in my bedroom, the ease and compactness of Carbo-Plus make it a really good alternative.