Wow - I can't believe someone else has read the Riverworld books! I just looked up book 5 (book 4 was apparently intended to be the last), and still can't remember if I read it or if I stopped at 4. My recollection is the same as your experience - the first ones are great, then it all falls apart.
Just please, don't watch the SyFy miniseries they made recently. I saw it cheap and picked it up, and it was horrible!!!!
Currently reading book 4 of the Prince of Amber series, but am almost done. Book 5's in my backpack waiting for me. On the other hand, book 4 of Scott Pilgrim is on the table in front of me. It's going to win that fight. :)