I never used one, but read up a lot because I was tempted to buy one and
decided not to do so because the device reduces KH to produce CO2 (oposed to
the manufacturers (sp?) claims it does so buy electrolising waterand let the
O2 react with the C in the block...).
This means fluctuating KH levels if replenished with waterchange, or no CO2
when your KH gets too low (ie, a variable where you have no control over).
It *does* work however, people have used it with good succes, although the
gallon rating for a max. of 500 liter tanks is reported to be somewhat
optimistic.
Of course all this is "hear say"knowledge, so please investigate
yourself....
Maybe you might be better of with presurised CO2, I just bought me a
complete set (used) for EU 80,-