I am currently using one. I agree with HoopsGuru it is very noisy out of the box. With a simple modification it is a very good skimmer for a small tank or used in conjuction with another skimmer. I bought mine on the web ($30.00) for a 20 gal
tank before I knew much about them. I now use it on my 75gal tank in conjunction with a Bak PAK and a canister filter. It skims almost as much as the bak pak. I do not use any of the mechanical filtration pads the skilter comes with and have filled the box with bio balls.
The modification is as follows:
Remove the vent tube that comes with it. By a good wood air stone and a small quiet air pump. Take a 35mm film canister(minimizes extra bubbles) and punch a small hole in the bottom(big enough for air line tubing). Place the air stone in the film canister. Put hem both inside the skimmer column and thread the air line tube out the bottom and to the air pump(use a one way flow valve to prevent potential backflow problems).
It now runs really quietly and skims very well. You can put a filter pad in if the bubbles bother you in your tank. I found that the bio balls do an adequate job of keeping excess bubbles out of the tank.
If you are on a budget this is a good option for a protein skimmer on a small tank(30 gal
or less) if used with another reliable mechanical filter.
Of course if budget is not a problem their are better options available.