I have very mixed feelings about this book, but feel compelled to give it 4 stars nonetheless. Why? Because what you can get here, you simply can't get anywhere else. The only other decent introduction to Classical Tibetan that exists (so far as I know) is that by Stephen Hodge, and it is much smaller and will simply not give you the depth of grammatical knowledge of vocabulary that this book can. Some reviewers have complained about the "Tenglish" approach, but I can't really think of any other way to present Tibetan grammar in a comprehensible way. Goldstein does the same thing, though less explicitly, in his "Essentials of Modern Literary Tibetan" (in all the transliterations) and it works for me.
Another reviewer commented that the book is overly pedantic in its detailed explanations and grammatical quibbling - well, what does one expect from a 700-page tome on archaic (more or less) philosophical grammar and vocabulary? You didn't think Classical Tibetan was going to be a walk in the park did you? In any case you can simply skip over the details when Wilson gets a little too in depth.
The major problem with this book as I see it is that it is fairly unbalanced. Meaning, in the first 7 chapters or so there are essentially no sentence/vocabulary exercises, leaving you to somehow (by rote, was my method) memorize some 150-200 terms that are introduced (and not easy ones - 'non-associated compositional factors' comes up, e.g.). This improves though, with quite a few exercises in the later chapters. This added context and required practice/effort really helps you to memorize the vocab and understand the grammar better. Presumably these were left out of early chapters so as not to discourage the student or to make it easier, but instead it just means you have lots to memorize without much contextual help - a big mistake, in my opinion.
Which leaves me at the final point, which is that this is a necessary book, I think, for anyone interested in Classical Tibetan. The field is simply too small. The only other 'intro' level books really are Craig Preston's "How to Read Classical Tibetan" series (two volumes so far, hopefully more to come), but these really aren't introductions. They presuppose thorough knowledge of how to read Tibetan and an understanding of its grammar, as well a fair vocabulary. He was also a student of Wilson's, so all his terminology and explanations etc. follow Wilson's style and terms.
In short: yes there are problems, sometimes it is a bore and overly pedantic, there are not nearly enough exercises for a self-learned... but you need this book if you want to learn Classical Tibetan. So get it and wade through it - it is worth it.