I have been a fan of Gesenius for a long time. While I knew there was updated information to consider, I thought his text was enough. But although his text is certainly still necessary, and necessarily covers issues that the book reviewed here does not, it is clear that one must turn to Waltke & O'Connor to reach a greater level of accuracy. Writing so long after Gesenius, they have presented new and accurate information based on the latest research, not only of Ancient Hebrew, but also based on cognate Semitic languages.
I have found the sections on Hebrew verbs the most enlightening and correcting, e.g., that the Pi'el stem is NOT intensive. Such intenseness must be detected from a combination of the lexical verb meaning, the stem, and the text/co-text. This book also presents other up-to-date information on the forming of nouns from verbs, etc.
As others have noted, the layout and presentation is great. It really can't be presented any easier, I don't think.
One must read Gesenius, for he presents very important information across the board, including much on phonology and linguistics not covered by Waltke & O'Connor. But to stick with Gesenius will leave one in error on some points. Waltke & O'Connor, simply put, make the corrections and make studying Biblical Hebrew exciting, and even much easier to learn.