First, I must say this is the only Modern Written Arabic (MWA) - English dictionary that the student of Arabic has to have. Others, al-Mawrid, for example, are useful as supplements, and contain new vocabulary, and there is a more recent German edition (5th edition) of Wehr published by Harrassowitz, but this book, German or English, has a standard of scholarship unrivalled by any other MWA-English dictionary. Middle East-published MWA-English dictionaries like al-Mawrid don't give the grammatical information learners of Arabic need, such as broken plurals, verbal vowelling, verbal nouns (masdars), let alone how verbs are used with prepositions, all of which Wehr tells the user.
Words are in root order, so maktaba (desk) <mktbh> and kaatib (writer) <k'tb> both are found under the verb kataba (to write) <ktb>. This really is the easiest way of ordering Arabic dictionaries once you've mastered your grammar, though an alphabetic order dictionary is a help when you're starting and occasionally even when you're expert.
This dictionary is NOT a dictionary of Classical Arabic (although Beeston in his anthology of Bassar bin Burd reckoned that Wehr covered the vast majority of the vocabulary of this poet of the 8th Century AD). For Classical Arabic, Lane (perhaps supplemented by Hava's al-Faraid) is essential. But Lane is useless for modern Arabic. And if you're reading mediaeval Arabic, you will find Wehr fills in some of the gaps in Lane.
This dictionary is NOT a dialect dictionary, though it contains many dialect words that have found their way into the written Arabic of Egypt, Iraq, etc. Arabs don't write colloquial Arabic (at least not in formal contexts) and dialect dictionaries are specialized (dialect-English dictionaries are often written in transliteration rather than in the Arabic script). If you need a dialect dictionary, get one. This isn't one.
The 4th paperback edition is quite a lot bigger physically than the 3rd paperback. The 3rd edition was 114 x 162 x 45mm (4.5" x 6.4" x 1.75") in size, weighed 0.65 kg and had tiny 5.5 pt print. The 4th edition is larger: 216 x 130 x 40mm (5.2" x 8.5" x 1.5"), weighs 0.8 kg and has 7.5pt print. This makes the 4th edition's print much more readable than the 3rd edition's.The 4th edition which is sewn-bound is also more robust than the 3rd edition which was perfect-bound - I'm on my 3rd copy of the 3rd edition while my 4th edition soldiers on after 10 years. However, the book is not really pocket sized any more (I still keep using my last copy of the 3rd edition as a pocket copy).
The 4th edition is expensive (it's cheaper in the US than in England, though). If you're in the Middle East, you can pick up Librarie du Liban hardback copies of the 3rd edition (they have larger print than either of the two paperbacks - about 8 pt) for rather less. But I'd advise students to get the 4th if they can afford it. If they've lots of money, perhaps get the Harrassowitz hardback - I've not done so. And if they have money and German, get the 5th edition (Arabisches Wörterbuch für die Schriftsprache der Gegenwart).
Since I wrote this review, I've bought the English Harrassowitz hardback, which has 8 pt script and a nice paper slightly less bleached than the SLS edition. Excellent if you're using the dictionary a lot, but a lot of money if you're not.