the book is especially useful when preparing for an examination, because it gives that kind of information you are looking for, by getting right to the point. Very helpful are the cross-references which can serve as guidelines for further searches. David Crystal gives detailed and exact definitions of the most important terms of linguistics and phonetics, but he neglects some of the key terminology of historical linguistics. Looking for definitions in this field of study, one should rather consult Hartmann / Storck:Dictionary of Language and Linguistics. However, particularly in the field of phonetics and phonology the book is an undefeatable source of information.
This dictionary is quite useful for university/college students once having passed the introductory courses. Reading any kind of "real" literature from science like articles taken from journals still can blast you with terms you can't really understand. I bought this book a few weeks ago and I'm using it quite often and it never failed to explain things. But if I was entirely new to linguistics I wouldn't gain much from it.