I had one on my first 29 gal and it actually worked fairly well. They will only pull out wet skim, but that is what I wanted. I wasn't trying to grow SPS so my shrooms and zoas didn't mind it. I only had two fish, a pair of percs, so the load wasn't high.
I got a cup of dark tea colored skim in 2 days. No I don't use them on my tank now, but how much do you have to spend? There are better, but get what you can afford and what you need.