I feel like I should start out by saying that I am an intensely dedicated fan of Darnielle's many outlets, whether it be his LPTJ blog, The Mountain Goats, The Extra Glenns, his contributions to Decibel, etc. etc. Let me say also that I am pretty sure most of the reviewers (though I haven't read them all) giving this a five star rating are similar in their positions.
I cannot blindly give this book a five, as much as I adore Mr. Darnielle. It was not perfect, as much as a enjoyed it. There were stumbles, in my opinion, where Roger became a little too repetitive, or where some things just seemed oversimplified. But as a whole, especially as his first book, I really enjoyed the book, and it was a quick read. I appreciated it even though I am not a well-versed fan of metal or anything. The character of Roger seemed to cover his bases enough that I could still understand what he was saying about the music--I think this is mostly in part not to his descriptions of the music itself, but to his feelings about it. Darnielle successfully creates emotional attachments which allowed me, as a reader, to sympathize, despite never having been locked up in a mental institution.
I think anyone who had even a remotely rough time in their adolescence and who turned to music to make their way through their troubles will thoroughly enjoy this book and be able to, in some extent, relate to Roger's troubles.