My 4-year-old loves this - he finds it hilarious, enjoys the rhyming text, and is very interested in learning about the subjects. I find the explanations well-pitched for this age group, easy to understand and a good starting point for further learning.
My only problem with it is the American-ness and slight superficiality of the Almanac part. This isn't a criticism of the book itself, but a drawback for readers who aren't North American and/or who want their kids to develop a deeper appreciation of the passing of the seasons in nature. The 'holidays' in the Almanac are the ones that are important in the US (Thanksgiving, 4th July...) plus some odd ones like April Fool's Day, with almost no mention of feast days like Easter that are widely celebrated in the UK (where we live). It also says, over-simplistically, that winter starts in January (and spring starts in April, etc), and gives the impression that winter = 3 full months of snow (which may be true in the place where the book comes from, but definitely not in most of the UK). And there's no mention of the change of day-length over the year, not to mention solstice and equinox, which are important to my family (though probably not to the majority of people, I realise).
That said, we love this book, and I especially appreciate the Nature and Science Fair sections, though Papa Bear's role as teacher of the little bears, with Mama Bear following quietly behind, does date it. I'm now on the look-out for some other book that covers the passing of the seasons in a way that works better for us (though I probably won't find one that my little boy likes as much as the Bears).