Since I am an international student studying in the United States, my perspective is slightly different from those of other reviewers. If you are studying for standardized tests such as SAT or GRE, this is 'the' ultimate authority you should refer to whenever you feel unsure about the exact meanings of the vocabularies you see in past administrated tests. If you practice on questions on analogy for awhile, for example, you would discover that the ETS (Educational Testing Services), the maker of GRE and SAT, also relies intensively, although not thoroughly, on Merriam-Webster Collegiate Dictionary definitions to design their questions. Only by referring to the MW can you truly understand why certain answer choice is correct, while others are not.
As a bonus, you also have exclusive rights to send inquires on difficult words to MW resourceful editors, a service I find extremely helpful. Although responses are slow (because of, surprisingly, numerous projects in Merriam-Webster Corporation), you will generally receive the most authoritative answers from the brightest people on the planet. As an ESL learner, I am deeply obsessed with the flexibility, inherent logic, and beauty of English language; all introduced by this enlightening tome.