Good science fiction. I'm not a sci-fi reader usually, so can't compare this to anything besides other Spider Robinson and Douglas Adams.
But what these have that kept me turning pages are the PUNS. The longest, most involved puns ever imagined. They are phenomenal. Well, and the plot. I mean, the stories are good and entertaining, but if you like puns even a little, you have to love these huge, intricate puns.