I started using co2 because of the feedback I got at the time. I also have a closed system with all my power maxed out to the point were I'm comfortable with it. So I figure these plants are going to need 'fresh air' or Co2 in this closed room (no ventalation)
I use a 13000btu portable airconditioner_ which cools the room to around 77-80 degrees F, with no ventalation. I use 4x 400 watt HPS lights with two 4x4ft ebbn'flo trays. The tanks are 50 pounders when filled at 1200psi. Which lasts me about 10-11 days. Regulator (110$) and infared monitor (600$) are used to maintain @ 1200ppm during light period.
Plants are 3 weeks into flowering and are 4-5 feet tall,.. in 4 inch squares of rockwool. Yes Yes, root bound right now, which im working on. But yo, most of then look very healthy.
I'm not sure yet how much the co2 is helping, But I figuare I already have it set up(to fill one tank around here is about 16-18$). I havent been seeing much growth in the last few though..