Before buying this book you need to ask yourself just one question. What are my language goals? If you want to pass a GSCE exam or have something to use for reference on holiday, then you can buy a much cheaper, smaller and lighter dictionary that will do the trick just fine. However, if you want to be an advanced Spanish learner, then you'll need a substantial dictionary, and this is the best I've come across, having tried several previous dictionaries of varying sizes.
One problem I have had in the past with dictionaries is the lack of words. This has over 300,000 words and thus realistically covers all the words that even an advanced learner would ever need. Also, there are interesting pages about SMS abbreviations in Spanish and how to navigate an e-mail programme, a must for modern communication. The CD is also a great advantage, and also allows you to toggle between European pronunciation and Latin American, which whilst not huge, is often distinct and thus this CD allows learners to get a feel for the different accents, or alternatively to select which brand of Spanish they are most interested in specialising in. Having said that Spanish pronunciation is very phonetic anyway, so the CD is probably less important than it would be in, for instance, French where the written and spoken language vary much more.
The main weakness of the book is not its content, rather the practical aspects of being such a large book. Its extreme thickness and weight make storage difficult and also makes it less than portable. Instead, this book is suited to private home study, not for being carried around in a school bag or a suitcase.
In short, if you take your learning very seriously and have language goals that stretch far beyond the "C or above at GCSE", then this is the dictionary to buy, but be warned it is very thick, and will take up lots of space on your shelves. Nevertheless, it is the best dictionary on the market to meet the needs of the advanced learner. Those with more modest language ambitions can buy a much cheaper, smaller and lighter dictionary that will work just as well for their needs, but for the advanced, this is the one to buy.