The first time we read this, the kids were entranced with looking for Anthony Ant in the maze of tunnels. The next time we read it the kids noticed a lot more of the many other characters, traced Anthony's route through the book, counted ants, worms, birds and many more.
My boys are 4 and 7 and it appeals to both. The younger one can't read yet but he loves the detailed pictures and looking at the animals. The older one always comes over to join in when it comes out. He likes tracing a path through the confusing maze of underground tunnels that Anthony is hiding in.
I'd say that this was somewhere in between The Hungry Caterpiller/10 Little Ladybirds and Where's Wally. Good for counting, good for looking, good for reading together. I'm just about to buy it as a birthday present for my friend's 4 year old.