Everything these firms tell you they'll do for you are things you can do for yourself, for free! Each credit bureau has an online dispute system. It pays to review your credit report regularly. You are entitles to a free credit report from each bureau (
www.annualcreditreport.com) or you can purchase one by going directly to the website for the credit bureau. You can dispute online and they have 45 days to respond. If something is changed they'll send you an updated report for free.
Be wary of firms that say they can remove negative items that are legitimately yours. They can't.