Accelerometers and GPS are a God-send for folks who run with their mobile devices. Everywhere I go these days, folks generally carry their Android devices or iPhones just for this purpose. Not only do these apps measure distance, many of them provide an approximation of the amount of calories burnt during the run.
However, apps also provide other uses. Tracking weight loss/gain. Finding out the proper diet including counting calories is also another popular use. These two functions quickly came to me because that's what I use my apps for.
How useful are they? Pretty useful but the right question to ask is how effective. Well, that depends on how hard you work. In trying to stay in shape or attempting to lose weight, it will take time and dedication. You may have the best mobile device on the planet or you may have access to, oh say, an unreleased 4th generation iPhone, if you aren't willing to sweat, no amount or quality of app is gonna help you.