Not a dog walker, but used ours since our dog was six months old. They have children, but also have two other walkers working for her which I think helps to ease the load.
The team is fully insured, DBS checked, first aid trained, the owner of the business is trained in animal behaviour, and they have access to a van for out of town walks. They take a max of five dogs at a time, and work between 8.30-3.30.
They have a website with an excellent booking system (which also REALLY helps us - this is something else to consider) but we also had to sign a number of forms and disclaimers about our dog's temperament, medical history, vaccination status, being off lead etc, as well as them "phasing" our dog into the pack. We had a meet and greet, our dog then met the owner's own dogs, and gradually walked first of all with one or two dogs at a time - this was all to test our dog's temperament and check she was suitable.
They are extremely good at what they do, love the dogs and are very thorough (hence why they're so popular) but it clearly takes a lot of effort, time and resource to be as good as they are.