I found one on sale for $20 and bought it for the hell of it, but as far as I'm concerned, it's pretty much just decoration in my room. I'm with you namvet25 and think it's probably not worth it.
My research dug up some credible users that stand by it and say it works and I did notice that many of the negative chatter was from people that have never really used the bags... they just blew hot air on the why's and how's of CO2 that "proved" the bags didn't work.
Then again, I found some credible users that said it did nothing for them.
So when I say "it's not worth the money", I guess I mean that IF they work, my guess is you'd need a lot more than one, circulation has to be just right and the bags HAVE to be placed right above the canopy for the CO2 to drift down to the plants.
That last part is the biggest problem of the bags, IMO. I have a long bay of CFLS with a wing reflector just inches above my plants. How the hell I'm suppose to hang a heavy ass bag like that in my situation is beyond me... not to mention that the bag is plastic and despite the low temps of CFLS, it's still a firehazrd.
So maybe they do work, but it would take more than one and only placed above the canopy... In the end, they are more of a pain in the ass IMO and not really worth it.