On the plus-side : You get a fair idea of the personality of the rocker in question, as he/she is speaking in their own voice, from their own perspective. Who knew that Lemmy Kilmister, may he rest in peace, was such a conservative old fart?
On the downside : Very often they just don't remember much of their past, alcohol and drugs having all but wiped out what brain cells there were to begin with.
Also, they tend to embellish the truth a bit, like the rest of us. (Ace Frehley "Oh no, sir, I had NO IDEA those S:es would look like SS symbols! I just thought they looked cool.")
A biography penned by an outside observer - preferably, as in this case, a journalist - when well-made - as in this case - therefore presents the advantage of looking at the facts from several different aspects, speaking to several different people, thereby improving your chances of getting somewhere nearer the actual facts.
Moreover, journalists tend to be better at phrasing, making the reading easier and more pleasureable.
The inconvenience (I told you! There is one to almost everything.) is that the object (here; the band) is viewed from the outside and you never feel really close to them.
Which is also a pity, as I'd be truly interested in learning more about the business machine Simmons - Stanley.
All in all; a fair read.