I agree you can do it yourself for free if you're willing to spend the time and energy. That's why we have this forum!
Credit repair services are regulated by the Credit Repair Organizations Act, plus some states have additional legislation and licensing requirements.
One thing to look for: they cannot take your money until AFTER they have performed the service. So look very very closely at how they define what they do for you. Most of them get around this by charging you a big "set up" fee (for doing nothing more than entering you in their computer) and then bill you the monthly fee for "repair" services performed for the prior month.
Keep in mind that they normally charge on a monthly basis. What incentive do they have for repairing your credit quickly?