One could say that the real protagonist here was the speed of light, and the characters were there to struggle with it and deal with its implications.
I really liked the first half of the book; then it dragged a bit; but then it got good again near the end. I did not see that ending coming.

And this was one of those books where I kept wondering how much my enjoyment of it is predicated on prior knowledge of the topics discussed. This is a hard sci-fi, and while the author does a good job of explaining everything, I still wonder whether it makes a big difference or not.