I have not used them, but skimmers are rather simple creations. Looking at pictures of these units, I see no reason why it should not be an effective skimmer for aquariums 55 gallons an under. The design is a basic venturi, with adjustable pvc output to control the waterheight inside the skimmer.
This design is a very common design and available all over the internet for the $100 price range or less. I would not personally use this skimmer, because the skimmer chamber is not large enough to be highly effective with a venturi design. However, if this is an upgrade for you and you are getting an extremely good deal, and IF YOU HAVE A SUMP, then you may try it out.
If you are using the hang on design, I can tell you will not be happy.