I was hesitant to buy this since I already owned Margoliouth's lexicon. However, I am very happy that I did.
The dictionary itself is ordered alphabetically, and it lists words by attested forms rather than their roots. This means that shin-causatives are listed under shin, hollow verbs like qwm are listed under qm, etc. This makes it easy to look up words you are uncertain about.
The notes and pictures scattered throughout every page of the Syriac-English section make it feel like an encyclopedia of Syriac history and culture. It's a joy to simply pick up and read.
There are extremely helpful grammatical notes in the dictionary, and a substantial appendix with a grammatical sketch. While the grammatical sketch is not detailed, it includes a lot of unique information you won't find other places, like diagnostic information that can help with parsing.
There's more! There's frequency lists, there's a section on greetings in modern Syriac, and of course the English-Syriac section - something that you can't find anywhere else.
This book is not a replacement for an academic lexicon like Margoliouth. Instead, it's something very different and so very valuable - a true treasure trove of information on Syriac language and culture.