First off, I the only reason I didn't give this book 5 stars is because, like others have said in their reviews, character development is kind of lax. In Baxter's defense however, the story is told in little vignettes spanning 5 million years. Of course you're going to have to leave old characters behind and introduce new ones. In my opinion, he does a passable - but not outstanding - job of character development.
You don't need a degree in physics or quantum mechanics to understand the concepts Baxter presents, but an "armchair" familiarity with these disciplines will help. Still, Baxter does a very good job explaining difficult concepts within the stories. The scope of the overall story arc is amazing and it left me with a very warm feeling and a little more optimistic opinion about Humanity's future. For a scientist come late to writing, Baxter has achieved far more, in my opinion, than most of his hard SF contemporaries.