Personally, this has been one of the most helpful books I have ever read. In a time of spiritual depression (what David calls 'the depths') on account of sin and the trouble that accompanies it, no other has given me more hope, peace, or encouragement than this one. There are a number of other great books on this subject, both old and contemporary (see Spiritual Depression by Lloyd-Jones, Personal Declension and Revival of Religion in the Soul by Winslow, and Losing God by Rogers). Each of them have their strengths and weaknesses but I have nothing but praise for the Forgiveness of Sin.
This book is so dense that I only read about a chapter or section at a time and each time I walked away comforted and frankly stunned at Owen's precision and more-so his ability to point the reader to the anchor outside himself - the promises of God. After reading certain sections I could not help but cling tighter to the promises of God that were made crystal-clear by Owen's refined methods of persuasion. This treatise is so concise that not a single page has any wasted material. It is one dense, continuous thought; suitably filling, with morsels so sweet and encouraging as well as with carefully measured stings of salty conviction and warning.
My wife would like me to go on record, saying: "If I had to lose every other book (besides the Bible, of course) in my library in order to keep this one, I surely would."
I highly recommend this book whether you are struggling in the depths or not. I think it would be impossible to fail to benefit from Owen, and from this work in particular.