I built one but it hasn't gotten hooked up to anything yet.
We did see one in action in Chicago on MWP's tank (breeds onyx percs, if you ever venture into the breeding forum) and he had his homemade setup going on a 24 gallon nano that was LOADED full of fish and softies. It was kind of ridiculous, but he wasn't having nitrate problems. I'm not sure of the specs of his, think it was 1 layer of 1/4" tubing 2-2.5 feet long on 2" pvc, but that's just a guess from memory. He had it putting out just a drop or two a second, as I recall.