It all depends on what you're using it for. It works pretty good to free up sticky rings, valves and lifters. If you're expecting it to increase your mileage or give you some kind of boost in hp, don't waste your money.
I've used it in little old lady cars that get carboned up. Gets rid of the pinging with one treatment. I have only used it through the vacuum booster per the directions. Be careful, you can do more harm than good.