these fluval 22g systems are good for short term, and/or small tanks, and/or low tech tanks that have limited plants or plants that dont require alot of CO2 like java ferns and crypto's. you can use standard 16g canisters as the threading is the same (and they are much cheaper). I have had 16g canister last 15+ fills of the passive diffusor included (1 to 1.5 weeks or so). one fill lasts 4-6 hours on my tank. the 16g canisters can be had on ebay for 30 for $30, so thats actually cheap at $1/week.
nobody would recomend these to be used with any other diffusor as the canister would empty way too fast and the "valve" is not reliable. also passive diffusion only keeps the CO2 balanced, or slightly over balanced therefore the non-recomends for high-tech plants. but they are cheap, cheaply maintained, and do the job they are actually designed for