I bought this book for my six year-old this past Christmas. She has recently fallen in love with Dr. Seuss books, and poetry in general. And this book, along with other Seuss books, gives her great joy. We read it together, out loud, enjoying the scan and ryhme of the words and sentences. And on that level, it's one of her favorites.
But after we read it, we talk about the concepts behind it, how "a person's a person, no matter how small", and how Horton realizes the inherent dignity in all life, regardless of whether or not it fits into our commonly held conceptions. This book allows me to open up discussions on race, and religion, and the external aspects of persons, and how often we judge people (sometimes unfairly) based on how they look, rather than on their actions.
I highly recommend this book for any schoolchild and parent to read together, reveling in the language and fun, and then use as a stepping stone to further discussions about life and personhood.