I realize that the signs aren't true ASL signs, but I find them easy to remember, and if my daughter wants to learn ASL later in life, I don't think learning some "wrong" signs will hinder her. In fact, many times the easiest way to teach them a sign is to assign a meaning to a gesture they already make.
Anyway, my daughter loves the book. It took awhile for her to start signing... the first one she did was "dog" (pant, or go woof woof... she says "a da!" but da can mean any number of things). Then we visited a friend with a bird, and I did the flap arms sign again, and something within her CLICKED. She flapped her arms whenever she wanted to see the birds.
The next day, she was doing the sign for "cat" and "fish". We watched Baby Einstein "Neighborhood Animals" video (the only one of the series that she shows any interest in), and when she saw a duck she went "quack quack" (sounds like guk guk guk) and started flapping her arms. She saw a bug that was flying and started flapping her arms as well.
If she sees a picture of a fish, she starts smacking her lips. I showed her the "one fish, two fish" book and asked if she wanted to read it and she smacked her lips for "fish".
We also use the back page as a "quiz" page, and I ask if she sees a dog (for example) and she'll point to the dog.
Granted the book doesn't have signs for eat, drink, etc... but using this book as a foundation to teach the baby that he/she can assign a sign to objects and animals, it's easy to build on from there.