Personally, I didn't like this book.
It was a staple of my earlier days in elementary school, and definitely cemented into the culture of my school. However, in recent years that has been a bit lost. The book its self is just okay, but to be honest it was never really, you know, it for me. I usually never read the book, and by third grade I have already outgrown the book. Sure, it has a lot of gross-out humor, but unlike other books I don't condemn that, because it always managed to sneak out a chuckle from me. That kind of humor is frowned upon towards lots of people, so this book easily is a, "Either way it could work."
If you like the humor and generally like the books, then read it, but if you're of a higher reading level, I recommend reading something else. To be honest there are so many other books within this kind of genre for early elementary school books, there's just too many to choose.
By the time I got in fifth grade (Like right now) everyone is reading Dogman, which makes me a bit sad, because Captain Underpants had a place in every child's heart. It was the book that defined a generation. A book that went pass that need to be fancy and serious, and took a generally more humorous approach.
That is what I like about Captain Underpants. It's what every kid likes into one book, the kind of collection of all that makes childhood fun.
If you don't like it, think it's ridiculous, that's fine. Although for the other people, read it. It's a classic and a famous book. While you have masterpieces like Fahrenheit 451 and The Odyssey in there, hidden in the hall of fame lays a small spot, for Captain Underpants.