I do most of my own work on my car, and what I can't do requires equipment that couldn't be transported easily, so I've never used a mobile mechanic. I do, however, know several people that have used them, and the general consensus is that they're like any other type of service person. Some are good, honest, decent, hardworking individuals, and others are scammers or lazy or simply bad at what they do. I would give you the same advise I would give anybody looking for any type of service person: ask for references, ask friends, shop around.