The Hagen Plant Gro system is just a store bought DIY. Its smaller and neater than soda bottles, and the little zig-zag bubble diffuser is kind of fun to watch for a while.
I was using one for a while, but eventually gave it up because it wasn't giving me as much Co2 as I wanted. Now I use two 2 liter soda bottles and feed it right into my filter intake. Cheaper too.
I would skip the canister and their mix and just get the bubble ladder. It's excellent for small tanks, I get 30ppm easily in a 10 gal, even though I have an HOB filter. You can just hook it up to your own DIY CO2, piece of cake.