These books are what the Captain Underpants books want to be--irreverent, subversive, anti-authoritarian, and actually funny. They scratch the same itch for kids of a certain age who think nothing is funnier than over-the-top, gross out, scatological humor, and yet they do not seem to irritate adults quite the same way as the CU books. I think this is partly because there is a certain kind of subtle meanness and schadenfreude that Sardine does not have, even though the bad guys certainly get their comeuppance left and right. Yet the good guys are not all goody goody either; they are space pirates after all.
I grudgingly let my daughter get the CU books from the library, but I willingly buy her the Sardine books as soon as I see the next one is available, and she has said several times that they are her favorite series.
Long live Sardine, Little Louie, and Captain Yellow Shoulder!