I found this book to be one of the better books that I have read in regards to IT. Too often I find that books do not offer a good blend of technical theory with practical application. In other words, either just told why to do something without being told where, or told to place check in said box without being told why as if I knew the material inside and out. This book didn't do that in my opinion. I felt as though the author had spent some time ensuring that the practical application was correct and each time I posed the question "why?" he was right there with a hint or note to give me some info.
Prior to reading this book I had used only ISA2K6 when it was pre-RTM (late Beta) in 2006 and early 2007. Since then I hadn't spent much time dealing with it but found this book to be quite useful in reminding me what I had previously learned as well as shedding light on some other issues that I had either not been able to find the answer to or had not had time to research (as many might agree, sometimes finding information on ISA isn't as easy as we would all like).
As far as those of you looking to get the +S on the MCSA or MCSE...yes I used this book for that purpose. I have taken the 70-351 and I did find this book very helpful. On some of the questions posed throughout the test I recalled some of the information provided by the author (and remember that bit about theory and application - helped).
Will I go as far as to say this is the definative book for ISA2K6, no. I haven't read the others. But I will say that if you use it or plan to, this book is worth the time.