I use the "boost buddy" Co2 bag. Co2 is obviously an essential element of proper growth, however, if you're doing a very small grow 1 or 2 etc plants, then it is unrealistic to install a co2 system. The risks involved in operating a co2 system are relatively high for unexperienced growers. So, there aren't many options for small growers. One option is the sugar/yeast method. It works fine, but does not last very long at all. Some claim 2 week life spans, but I never got 2 weeks out of a 2 liter with sugar/yeast method. So, I bought a co2 bag. My reasoning is, you HAVE to do something to up your co2...even if it is just marginal at best. I was able to toss the bag in the tent and stop using the stupid sugar/yeast 2 liters. I haven't noticed any difference, or any signs of a co2 deficiency..which i guess would be death.
Anyway, not sure this helped. I'm a new grower so I don't have any REAL comparisons to go from. However, not having to replace 2 liters every week is worth 40 dollars without doubt.
Edit.
If the bags were more expensive, then I would say no way. But at there current price, it's worth tossing in.