Isaacson manages to portray the personality of Leonardo well-enough. His style in this book is kind of less riveting than "Steve Jobs" book of him.
As for Leonardo, his main characteristics are extreme creativity, curiosity, being a great observer, perfectionist and polymath. On the other hand, it was surprising to see that he was a great procrastinator and was infamous for not finishing what he started. He jumps from topic to topic and pursues knowledge just for the sake of knowledge which does not make sense at all. Had he managed to focus his attention, he could have contributed to the humanity much more. So, to me, by just comparing his potential and performance, Leonardo's life was a "failure".