I'll admit I've never read any of the Captain Underpants books by Dav Pilkey, though I am a huge fan of his books Dogzilla and Cat Kong. According to this article the Captain Underpants books are sixth on the ALA's list of most frequently challenged books of 2002.
Of course I can't provide a link to the above list because ALA managed to completely ruin their website last spring when it underwent a huge overhaul. I tell you, it's freaking terrible to try and find anything on that site. I searched using the phrase "most challenged books 2003" and got links to conference proceedings. Spleesh!!
AT ANY RATE, it has been my experience that the best way to get kids to read any particular book is to tell them that they aren't allowed to read it. My 10 year old niece has been forbidden to read the latest Harry Potter and she is dying to get her hands on a copy. My 8 year old, on the other hand, has her own set of all the Harry Potter books and she has yet to finish Harry Potter and the socerer's stone.
But regarding Cpt. Underpants, why can't kids just read fun stuff, eh? What, like none of us adults ever read comic books? Guh, it's time parents quit putting up roadblocks to interaction with their kids.