Like virtually all of the DK books I have seen, this one is exceptionally well done. We are using it for gr. 3 homeschool biology. It covers each of the animal groups (mammals, reptiles and so on) quite thorough and is very engaging. The pictures are bright and fun and the text is in small, easily digestible chunks. I think the amount of information is just right for my daughter, though I can see that younger kids would probably find it quite fun as well. I'd imagine kids older than gr. 5 or so would like it too, though they might be looking for more detailed information.
I've been surprised at how interested my two boys (5 and 7) have been in this book. The animal descriptions are informative and not condescending and the pictures are informative and vibrant. It has also been a great introduction to indexes. They love looking up the animal's name in the index and finding the page number to turn to. Another great DK book to add to your collection.