I am the British owner and operator of a scuba-diving company in Bali, Indonesia and we have several copies of this book available for dive staff and divers to use as reference.
That said, this is more than simply a 'reference' book. I don't think I've ever opened it to look for one particular fish/nudi/whatever and not found myself flipping through the pages. They've even included Mola-Mola (Oceanic sunfish)!
The pictures are well-shot and very clear, whereas in some "ID" books the photos are a little unappealing.
The info for each entry is succinct and useful. The "zoom" pages are informative and, while there's no 'dumbing down', are relatively easily understood by people's whose first language is not English which is useful for us.
No book can be all things to all people but an indication of how frequently the books are used for reference is that we have had to get them rebound several times. (Nothing against the quality of production, they're good quality books!)
If you're particularly "into", say, nudis, then buy a nudi book but as an overall ID book, with a fair amount of extra info thrown in, I'd recommend this.
A quick survey of my dive staff produced comments that included "very good - please buy more!" to "extra-emely useful".