I put one on my water heater (foam sheet) and insulated the couple of feet of bare pipe between the water heater and the wall where it enters the house. I couldn't see a discernable difference in expense, but after draining the tank by filling up my whirlpool bathtub during the winter I seem to get hot water available more quickly. My wife also thinks we get warm water out of the faucet at the other end of the house a little more quickly.
Remember, that I live in Arizona and much of the year my cold water is hot enough that I don't need to turn the hot faucet on. I only put the thing on my water heater because a friend who was moving out of the country gave it to me before he left. I figured it couldn't hurt. 
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