I had one of those Hagen units. I found it fascinating to watch the bubbles flow back and forth up the ladder. You can actually see the buddles shrink in size as they get to the top and are absorbed directly into the water..! It was like one of those lava-toys for stoners from Spencer's Gifts at the mall that you turn upside down and watch the little blobs of liquid drip up and down and go in spirals and move little wheels and things. 
However...
After a while I got tired of looking at that big ugly chunk of plastic in my nice purty little planted tank, and instead I connected the CO2 tube to the uplift tube of my undergravel filter powerhead. I poked a hole in the uplift tube about 2" below the powerhead, jammed in a small airline-elbow, then connected the CO2 tube to that. This does two things: 1) works like a bubble counter, and 2) pulverizes the bubbles into a fine mist that gets instantly absorbed into the water - all you see is a barely visible fine poof of bubble mist shooting out of the powerhead output which disappears by the time it gets like 3" out of the powerhead.
And don't forget your check valves...!!!
However...
After a while I got tired of looking at that big ugly chunk of plastic in my nice purty little planted tank, and instead I connected the CO2 tube to the uplift tube of my undergravel filter powerhead. I poked a hole in the uplift tube about 2" below the powerhead, jammed in a small airline-elbow, then connected the CO2 tube to that. This does two things: 1) works like a bubble counter, and 2) pulverizes the bubbles into a fine mist that gets instantly absorbed into the water - all you see is a barely visible fine poof of bubble mist shooting out of the powerhead output which disappears by the time it gets like 3" out of the powerhead.
And don't forget your check valves...!!!
