I agree with Max.
If you dont have all the variables in place with a substantial sized grow, you are going to waste alot of money.
On the other hand if you are just doing a few plants for personal use, then using the fermenting process, dry ice or even burning a lamp in the room will increase the C02 level slightly and your plants will use it, so its not a waste of money. I have had success with a box I built back in 'nam, its was roughly 800x800 with 1x 400W HPS and with 2x 2L coke bottle in there my growth shot up noticeably.
I always believe that you have to try something to know how its gonna work, you cant rely on other growers experiences cause all our set ups are different.
Let us know your findings.