How should you use the rankings? For initial research and data mining. And maybe a little reputation checking if you are lucky enough to receive multiple acceptances.
First for that initial research: Let’s say you are looking into the schools you will apply to. You recognize that your academic qualifications are an important element in that decision so you want to know average test scores and GPA for the different schools. That data is conveniently found in US News rankings . For MBA's, you can also find it at BusinessWeek and the Financial Times.
US News' rankings also allows you to rank or select programs by a limited set of criteria. For example, you can filter medical schools, business schools, and law schools by specialty, tuition, or class size.
MBA's have more options. If you are concerned about return on investment, then the new BW ROI rankings are enlightening as well as The Financial Times’ and Forbes’. If you want to know what students thought of their MBA experience, turn to BW and The Economist/Which MBA. Perhaps you seek a ranking of European programs because you intend to study in Europe; The Financial Times provides a ranking of European schools.
Again, none of these rankings is a substitute for research, but they can be a launch pad.
In a nutshell, use the data in these databanks to help you rank programs according to your values, preferences, and criteria while taking into account your qualifications, goals, and personal preferences.