I was hooked by "To Your Scattered Bodies Go" a few weeks ago, and have been blazing through the rest of the series. I just finished "The Magic Labyrinth", the fourth book, which completes the major arc that brings the "lazari" of the Riverworld face to face with the people responsible for bringing them back to life on this new world.
The first installment was largely the story of Richard Burton, and introduced the questions whose answers might lie at the source of the river. The second book was largely the story of Samuel Clemens and his dream of building a boat that (among other things) is the best hope of travelling upstream to the source of the river. As the river windws around the entire planet, most of the third book was spent in transit.
The fourth book brings us in sight of the goal and unlocks its secrets (or at least some of them). It's a major payoff after a series of small revelations and a long journey with these (now familiar) character. Well worth the time.
I'm working on the fifth book "Gods of Riverworld" at the moment and will review that in a few.