The list of Advanced Learner's dictionaries is as long as my arm: Oxford, Cambridge, Collins...---and now there's Merriam-Webster's. I guess it was about time: the market for this kind of thing is huge. It is not surprising that M-W finally wanted its share of the pie.
Well, M-W did not re-invent the concept: If you know the layout of one, you know them all: headwords are in bold, pronunciations are in the International Phonetic Alphabet, definitions come next, usually followed by one or two examples (in blue) to clarify the meaning of the word in question. Usage notes and illustrations are interspersed throughout the book. Like the Oxford Advanced Learner's, M-W highlights approx. 3000 basic English words as the most important words a learner needs to know. Also included in the book: 16 pages color art (again, most dictionaries have those), an English Grammar review, often confused words, spelling rules, a handbook of style (mostly punctuation, capitals and italics), weight and measures, e-mail and letter writing, etc.
The dictionary covers both British and American English. But the focus is definitely on the latter: in spelling and pronunciation, American is the norm (which is fine by me: it is an American Learner's, after all!). The definitions are written in a clear and simple language (as far as I can tell). Entries are up-to-date. Last but not least, a free eBook download is included (but not yet available as I'm writing this).
What is mssing is an accompanying version on CD. The online edition makes up for it, though.
But the nagging question remains whether there is a compelling reason to buy it... in all fairness NOT if you already have one of the latest Advanced Learner's available out there: Oxford is still the gold standard (a bit overrated I think), and Cambridge is getting better and better with each edition.
But make no mistake: M-W's Advanced Learner's is an excellent dictionary and a worthy addition to the Advanced Learner's Dictionaries market. It is quality throughout. If you are thinking about getting an ESL dictionary (especially if you want to focus on American English), you may want to give it a shot.