Though I can see what others might have found frustrating about this book, I absolutely loved it.
Mitchell does something really fascinatingly bizarre in the way that he ties together these stories within stories, letting them loosely hang together without creating the strong links we've learned to expect as readers. He teases us, letting us hunt for clues and run up dead ends and leaving us ever so slightly unsatisfied and wondering about how it all fits together.
All this would be irrelevant, of course, if Mitchell wasn't also a fantastic story teller. All of his characters are brilliantly realized and his ability to write within every genre he looks to is amazing. If Cloud Atlas were nothing more than a book of six short stories, it would be worth the read. As it is, it's so much more than that.