I started using one of these last weekend. It seems to be working OK in my 100 litre, but I am experimenting with different recipes of yeast and sugar at the moment. (I am aiming to get a good supply for two weeks before changing the mixture, but some say it will last 3 weeks if you get it right.)
I also put a drop checker in the tank to check the CO2 levels and this has been showing constant green (which is ideal).
The yeast that was supplied with the unit is dead so it didn't work at first, but it is easy enough to buy your own and the stabiliser included in the pack is just bicarbonate of soda. So there is no need to buy Hagen's own expensive stuff.
Now I am trying a mix of 1/2 teaspoon Allinson's fast acting bread yeast and 1/2 tsp bicarbonate with granulated white sugar and it has been putting out a constant flow of 14 bubbles per minute and the ladder/diffuser works fine. You can actually see the bubbles of CO2 dissolving in the water before they get to the top. The ladder is also invisible in my tank hidden behind some Hygrophila. This mix has been going now for 4 days.
My pH also came down from 8.0 to 6.8 and now seems to have settled on 7. My algae is also disappearing.
So all in all I am very happy with it, but it is still early days. This is my first venture into CO2 and I am thinking of getting a proper system for my 200 litre.A picture of the 100 litre is here in case you want to see how planted it is:
http/myaquariumlog...ped-tank-1.html
The ladder in the tank is here:
http
/myaquariumlog.blogspot.com/2011/04/hagen-nutrafin-co2-unit-in-action.html
Cheers.