Well,

the popular opinion on Google was: It works but due to the "waterfall"
type effect this filter has much of the CO2 escapes back into the air
immediately. You have to pump much more CO2 into the water to achieve
the same results, because of the losses.

Search in rec.aquaria.freshwater.plants for: "CO2" and "waterfall"

The first result (I think) pretty much explained it.

..... sorry about that. hehe

my aquarium page, info and pics at:
www.geocities.com/spiral_72/Spirals_page.html