I read the first edition of Hancock's book about 9 years ago. In spite of its immense size, I read it in one night. It's a fascinating look at the mysteries of ancient culture. In this edition he doesn't update the main text, but provides some updates and answers to his critics in a new introduction and some new Appendices. Since he first wrote this book, the paradigm has been changing. More scientists and researchers have suggested a greater antiquity in the Americas, larger populations and more pre-Columbus visits. These may explain many of the mysteries Hancock discusses, much better than "Atlantis" or some lost super-civilization, and he admits this in the new intro. However, he does not think these new finds completely eliminate his ancient super civilization. And indeed, there are still many questions, but multiple pre-Columbus visits and settlers is still the best answer.
He also now recognizes that "earth crust displacement" probably isn't the best explanation for the end of the Ice Age and submersion of lands. Sure, readers wish he would have put forth a new text, but with his addendums, this book is still of great value for those interested in ancient history.
Ironiclly, there are two irrational lines of thought that promote the isolationist models of the New World: 1. Those who think that natives became advanced completely on their own and any claims of interference are "racist"; and 2. Those, whom for whatever reason, still can't grasp that mankind has been traveling the seas long before Columbus.
See also 1491: New Revelations of the Americas Before Columbus, 1421: The Year China Discovered America, Who Was Adam?: A Creation Model Approach to the Origin of Man and The Island of Seven Cities: Where the Chinese Settled When They Discovered America for some recent developments on these topics.