Written in the mid nineteenth century, the style was noticeably different from modern expositors. It was engaging; but you really have to be paying attention the entire time (i.e. this book is not one of those you can read with a toddler running around being all cute and cuddly and handing you things - you'll need to be in your study, alone).
So if you can get the alone time, it's definitely worth the read. Even if you can't bring yourself to the same conclusions Willson draws, he gives you much to think about; and he provides very adequate Biblical text, and logical rigor to support his exegesis throughout.
Whether you agree with him a little or a lot, this book will absolutely help you think more acutely on the topic of civil government and its Biblical establishment and limits - and those pesky but very important applications we must then draw from the texts of Scripture.