I suspect its similar in operation to the watt wizard I have. It will save but the realistic savings will not likely ever off set the purchase cost!
I still watch auctions and garage sales for them though. If you pay under $3 they are cost effective!
For refrigerators, freezers, water pumps, heat pumps, and air conditioners they do work but not as great as they advertise!
If you get the chance to buy one used for cheap get it! You can spend more money on worse stuff!