This book I bought as part of the MCDBA boxed set from Syngress, and it's the only one of the batch that's really worth your money.
The questions in the book are very hard, and although the format is a bit different than in the real test, it's my personal opinion that these are harder than on the real test.
Each question has elaborate answers telling you both why the correct answer is correct, and why the incorrect answers are incorrect. Here, I learned much about SQL Server that I didn't know before, so with this book, you not only test your knowledge; you also learn while you do it.
More than one author has contributed to this book, and it's my impression that the section(s) on the design test are more thoroughly worked out than the ones on the administration test.
My only real gripe with this book is that some questions (most in the administration section) seem to have erronous answers, but it's not many.
This book will not teach you SQL Server, but if you know the product already, this book can help you prepare for the test(s).