The bad thing about using CO2 is that it's almost impossible (in my mind) to know you're not venting the grow room air too fast for the CO2 to get to your plants.
One commercial grower I met on another web site uses air conditioning and CO2 (from a tank) in his grow room, and does not vent his grow room except for a few times every day. He does not have a continuous vent fan running during flowering. And he's made a living at growing his whole life.