I like Wallace, but this book wasn't as helpful as Mounce's (the previous book in the series). I was MUCH more helped by the DVD lectures that are paired with the textbook (although, he gets on tangents a lot on those, haha).
Don't get me wrong, as a reference guide, it's super helpful, but not so much as a textbook. I think the organization and presentation need some serious overhaul for it to be a better textbook. For example, for being a textbook, he needs to devote a whole chapter right at the beginning to English Grammar, there were grammatical concepts my poor Government education didn't prepare me for, and neither did Mounce's book, that he just assumes you know.
Also, there needs to be more continuity between the chapters, if more than one grammatical category has the same syntactical rule, then maybe have a chapter covering common syntactical rules. As I made my way through it, rules would start to sound vaguely familiar, and then I realized it's because three other categories could function syntactically that way.