I don't really know what I expected when I started reading this book, but I found it both interesting and disappointing.
The Stefan Diaries chronicles how Stefan and his brother Damon became vampires. This book has nothing to do with the actual Vampire Diaries book series, but rather The Vampire Diaries TV Series. From the TV series (Season 1) we all got glimpses of Stefan and Damon and their relationship with Katherine, however I found that what we learn in the tv series is not congruent with what happens in the book, but maybe I should just watch Season 1 again and re-read Origins. It is good to know how the relationship between the Salvatore brothers and Katherine developed, and how it is that they became vampires. The incongruency is what disappoints me, I wish that L.J. Smith had written this from the point of view of the books as something of a prequal.
I won't say too much as I don't want to have a post filled with spoilers; so this is all I will say: This book gives a great back story and history of the Salvatore brothers from Stefan's perspective. It is vastly different from the book series, but is entertaining none the less. If you are a hardcore fan of the books, then don't read this; however if you love both, then go ahead and read this book, it will fill in many gaps.
I found it gripping from the first sentence so this book is spectacularly written, just remember that it should not be looked at as part of the original book series.