Many people are. They should be. It is VERY easy to be the victim of an auto repair scam. The easiest way you can keep this from happening is to stay informed. If you have taken your car in for repairs or maintenance then you should read up on what they will be doing. Spend some time on this site learning more about what is going on with your car. This will make it much easier to know what the mechanics are doing with your car.
If you can ask educated questions about your car then the mechanics will know that you are paying attention and you have won half the battle. The easiest people to rip off are the ones that do not know anything about their car.
Watch this great video that demonstrated this point perfectly.