I bought this in conjunction with Claire Rayner's, The Body Book - partly because that's the book I remember from my childhood and also because it deals with a wider range of body issues in excellent, child-friendly detail. However, it's the Let's Talk book that grabbed my 6 year old's attention. He loves the Bird and Bee characters and has been fascinated to learn about where babies come from. He had been asking questions about his body and this was just a really useful tool to help me explain things.
As an adoptive parent, I also love the fact that the book acknowledges so beautifully that there are all different types of family and talks about emotional issues around friendship and growing up.
A fun book for him and a useful one for us. I'm sure we'll keep coming back to it as he gets older too. Would definitely recommend.
UPDATE: A little while on and this is still his first book of choice, especially when reading alone.