How useful is the service of google ad words?

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Did anyone have a business before, or currently running a business? If you did, have you ever used google ad words to advertise the products you trying to sell? I just want to know, how useful is google ad words in your opinion? Do you think that service really help you a lot, or it only cause minor effect? My business partnet proposed me to use google ad words, but I am considering how useful it gonna be once we used it. Because the price isn't cheap due to my economic status right now.