The next one is awesome, but they go steadily downhill after that, alas. Just so's you're prepared.

Ian may not like them, but I think they're an excellent read.

I couldn't finish the third book when I was in high school, but when I read through the whole series a few years ago, I really, really enjoyed it. Be warned that books four & five are fairly cerebral and short on car chases. ;)

Great series; the movie they made from it was amazingly bad.
There were also some short stories about Riverworld written by various authors in a couple of anthologies, some of those were good.

Maybe I should have been clearer: To Your Scattered Bodies Go and The Fabulous Riverboat are, to my mind, both five-star classics. The Dark Design is three-and-a-half to four stars. The Magic Labyrinth is three stars, with a one-star ending. Never bothered with Gods of Riverworld.

As a whole, though, I love the series, and generally love Farmer.

I recently picked up "To Your Scattered Bodies Go" to re-read after I watched the movie "Riverworld" (available on Amazon) that combines the plots of the first 3 books. It's not a bad movie. Although it condenses quite a bit, it does stay relatively true to the spirit of the books. You might want to watch that after reading.