As mentioned, Google has many tricks and ways to make money and sometimes experience is the only way to learn. If you use a professional adwords company such as Add People they should have experience within the company that gets you away from irrelevant clicks and hopefully into the profit. Google has lots of useful tools such as:
If these are used correctly they should help to create a successful adwords campaign. The tools are free if you know what you are doing and should definitely be looked into and studied.
However, using a professional advertising agency is definitely an option. You wouldn’t try to cut your hair yourself or do your plumbing yourself, so why try advertising if you aren’t a marketeer. You could have a go at AdWords but may do a botched job or an just an adequate effort. Most agencies can use the tools quicker than you will be able to and it will save the time of mananging the account too, so you can get on with running your business.
If an agency-customer relationship is good then it should be a two way relationship where the agency and customer discuss campaigns and business plans and performance and then action the necessary account changes to see continuous improvement. What needs to be remembered here is it is the responsibility of both parties to make sure it works. You are paying for expertise and also time so as a business make sure you utilise the time that is being saved usefully.