I don't normally read biographies. When I try I always get bored and stop. This one went better for me though because in addition to being the story of Joss Whedon it is also the story of some of my favourite TV shows and movies of all time: Roseanne, Toy Story, Buffy the Vampire Slayer, Angel, Firefly, Dr. Horrible's Sing-Along Blog, Cabin in the Woods, Much Ado About Nothing and The Avengers (1 and 2). To say Joss Whedon has made his mark on popular culture is a vast understatement.
The author had fairly open access to not only Whedon and his wife, but almost everyone in his vast army of friends/colleagues (often the same people). As a result the book is full of great details about the genesis and making of many of Whedon's projects. I also enjoyed the details about how Whedon and the people he worked with were pioneers in engaging with audiences via the Internet.
Plus, the foreword is written by Nathan Fillion, which is just awesome.