Having read about half a dozen biographies on Che Guevara (written by a myriad of authors with different perspectives & objectives), I can confidently say that it is Castaneda, an eminent political scientist, who has written the most elegant and constructive evaluation of the life of Che Guevara. Although the book lacks the baroque detail of, say, Anderson's or Taibos' acclaimed biographies, Castaneda makes a moving and profound analysis. And unlike Anderson, who, as a somewhat mercenary journalist, comes across as calculating and contrived in his so-called "objectivity", Castaneda's sincerity shines like a beacon.
The only negative point of "Companero" is that the first half of the book appears to be somewhat rushed compared to the dazzling second half. Castaneda clearly prefers to write about Che Guevara from 1959 onwards. However, this criticism does not detract me from giving this book the five stars it deserves.
Because "Companero" is more of an "evaluation" than a full-blown biography, it lacks the detail of Anderson's, or the profound intimacy of Taibo's, books, so if you have time, read at least one of these first.
Another book I would highly recommend is the newly-released "Evocacion", written by Che Guevara's widow, containing a selection of previously-unpublished letters, including a heartrending, prophetic and immensely revealing poem written to her before he died. I can only presume that Castaneda, Anderson and Taibo's biographies would have been quite different had the authors come across this material at the time of writing their books. In fact, I suspect that Che Guevara's widow decided to finally release it after reading these biographies (notably Anderson's) - in order to close the circle once and for all.