You probably know OpenTable, THE site for booking tables in restaurants around the USA. As TechCrunch mentioned, there is a big market for booking and on-line appointment scheduling apps in many other areas and industries.
The disadvantage is low flexibility of such systems. The want you to do things your way, you really need to follow the way they do it so you can be listed on their site. The perfect appointment scheduling system should be much more flexible and customizable, allowing you to set your own rules and accommodate the way you know your company and business operates.