I had high hopes for this book. Too bad it didn't live up. Once you see how skinny the book is, you will start to be concerned.
First, what the book does well. It gives a nice general overview on how to translate d20 Modern occupations, skills, and feats to less modern eras. The book also does the best job yet for showing how to have a d20 Pirates/1600's setting, complete with a sample adventure. The various advanced and prestige classes (Explorer, Musketeer, Shaman, and Sorcerer) for that era are also well done.
Sadly, once you are done with the section concerning 17th-century adventuring, the book drops down in value fast. Instead of giving general guidelines for the eras in question, d20 Past force feeds two campaign settings that are only moderately described. (It gives 24 pages for 17th Century, but only 16 for "Shadow Stalkers" (Victorian) and 15 for "Pulp Heroes" (1930's)) Outside of the rifles rules that were presented in the first chapter of the book, there is nothing to recommend getting this book instead of prexisting d20 books like Forbidden Kingdoms from OWC or Masque of the Red Death from S&S. The adventure models are weak for those eras, and the prestige classes are nothing special.
With nothing on how to deal with the Wars of the eras (Napoleon, American revolution, American Civil War, WWI), and the exceptionally-poor-for-WotC artwork in the book, the only reason to pick up this book is if one wants to do 1600's settings using d20 Modern rules. All others should look instead at the books I previously mentioned.