I enjoyed Goldman's book, but you really have to use your judgment when reading it. For whatever reason, Goldman was fond of telling us John's thoughts. Wow, the guy was psychic! So just look for all those passages where it starts, "John thinks..." and you know the rest of that paragraph is all fiction. Also, when he describes in detail what John's penis may or may not be doing at any given moment in terms of flaciddity, you can pretty much write that off as well.
On the plus side, Goldman did numerous interviews and exhaustive research. The book was hated when it came out, but many of his shocking revelations-- such as John's use of heroin-- have stood the test of time and are now an accepted part of John's past. I found it interesting that people who were savaged in it (such as Yoko) never sued. Was she really taking the high road in refusing to take any action, or was she aware that the facts were so well researched that she would lose a defamation of character suit?
Like any work, I try to fit it into the larger picture of what other people have said and how they acted. For example, one of the most wounding things to Cynthia was when Yoko made her post-John's death announcement, the statement was signed "Yoko and Sean" and Julian's words were confiscated for Yoko's own use. So I get to chose who to believe: Cynthia or Yoko. I put it together with everything else I've heard, and just decide-- I hope while also keeping an open mind. I don't and won't know for sure, but I can make an educated guess-- fully aware it's just my best shot at assimilating the facts.