I agree with most of what's been said, so I'll just outline it from a perspective of someone who learnt Japanese in a communicative environment, and who is now looking at it from a linguistic perspective.
1. As per the title, the book is a reference grammar. This means that it is *not* intended for people wanting to simply learn from it. It means that it is *not* intended for beginners students. Rather, it means that the book is intended for two types of people; a) advanced/intermediate students/speakers of the language who are interested in having a *reference* to the lexico-grammatical forms of the language. b) Linguists who are interested in understanding some forms of the language for whatever reason.
To say that the book is 'learner unfriendly' goes against the whole point of the book; you simply cannot rate this book 1/5 because of this! The book is intended as a reference grammar!
2. The book is very extensive. I haven't come across any forms in the language that are not in this book. It also covers a lot of archaic and non-productive forms, which is helpful for people translating older or more formal texts. Despite what the author says in his defence, it is quite exhaustive. Yes this is daunting for some, but then you should obviously consult a different work.
3. The use of transliteration. The book doesn't use native Japanese characters because it is aimed not only as a reference for advanced students of the language, but for linguists who may not have time to learn any characters, or who may simply need to look up one form.
4. The book is accessible to linguists, and despite what has been said, is approachable from 'the middle'. You do not have to start at page 1 in order to use it.
I only have two criticisms:
a) Format. Sometimes, the lack of headings/bolding/boxes/etc makes it difficult to quickly locate specific forms or to find paradigm examples. This means that you have to sometimes read a few pages to find one construction.
b) Terms. The terms used by the author are sometimes a little idiosyncratic to the older schools of Japanese grammatical theory. Even though there is a 2004 edition, it seems that a few terms could be updated to more modern universally-accepted grammatical terms.
Other than that, I can definitely recommend the book!
If possible, I would give it four and a half stars.