These books are hilarious and smarter than they seem
My son, who's almost 6, has worked his way through 8 of the Captain Underpants books so far (we read them aloud to him). He LOVES them, and I enjoy them too. They're ridiculously rude, but that's part of the fun! The Common Sense review says that no one would mistake them for great literature, but in fact they have a lot in common with great literature; particularly modernist literature, which explodes chronology and is radically honest about bodily functions. They explore questions about time travel and history in a clever way, each book building on the previous ones. They're full of running jokes and mad surprises. My only concern about them is that they have a terrible approach to gender issues and a certain streak of homophobia: there's an episode where some footballers are tricked into behaving effeminately, for instance, and this is considered a humilation. However, I understand that Dav Pilkey's no-nonsense approach is meant to appeal to kids who might otherwise find that children's books don't reflect the (awful) truth about elementary school. I think these books are so much fun. My recommendation would be to dive in, read them aloud to your children, and use the problematic parts as an opportunity to talk about issues around bullying / gender / homophobia with them.