I am not going to appraise the value of that book. It is enough to say that for the past hundred year (exactly!) the grammar has been evaluated as a fundamental work, a monumental book etc. No grammar of Ethiopic could replace it up to now. There is no Semitist, no scholar interested in Hamito-Semitic linguistics who wouldn't praise that book. It is the largest Ethiopic Grammar both in German (original language version) as in English (translation of Critchton). It is a must for a person seriously interested in Ethiopic (I mean, Ge'ez) language.
Besides, the paperbook editon, although it has its faults if you are going to use it permanently, is quite clear and handy reprint.
I have only one warning for the students or persons who have no contact with that grammar: you have to know that the author uses original Ethiopic script (26 letters each having 6 variants, if you have had to do with the Arabic, you know what I mean) so you must either to digest very carefully the grammar (the script is not showing double consonants and the rules for the semivowel shwa are confusing), or, which is safer, to start with Lambdin's Introduction to Classical Ethiopic (Ge'ez). Besides, Ms. Lara Burton has a quite nice page dealing also with Ge'ez (giving a vocabulary list for 15 lessons of Lambdin's grammar, putting some links to the original Ge'ez texts, some papers about Ethiopic phonology and writing etc. [..]