I remember my younger brother being keen on these when he was younger, so we got this for our son, who is just over 4 and a half for christmas.
He absolutely loves it. He spends ages sitting and looking for all the people and the other little items. There isn't much of a story, but it has really taught him patience, which has allowed him to be more attentive to puzzles and other games that require the same skills, such as looking at things in more detail.
I think if your child enjoys books already and is happy to sit and look at books without them always having to be read to them, this book would be ideal. If not, this book may well be a way of encouraging your child to learn to do those things.
I also think there is some mileage in these books, and you can come back to them again and again and see new things.
I think we'll be buying more of these in the future...