My son (first child) was not enthusiastic about the book, but he DID want to learn to read. So we did it and he was a fluent reader at 4 1/4. (We started the book at about 44 months old).
My daughter (who is 3 years younger than our son) wanted me to teach her when she was not quite 3 1/2. We tried it for 2 lessons and she just wasn't quite ready, and it was Xmas so I was busy. I put it away until January, and then she took off with it. She finished the entire thing before her 4th birthday b/c she loved it so much she would do multiple lessons a day in the early part of the book.
We DID do the writing, but not with pencil and paper. I just got out a magna-doodle and let them use that without any lines. It really did help reinforce the sounds the letters make. My daughter was reading Henry & Mudge, Frog & Toad, and much more at 4 years (and I mean JUST 4... like, 48, 49 months)... she is still an avid reader and currently has about 4 different books going. Her latest pick is the Indiana Jones Trilogy (based on the movies) that she picked up at a closing Borders a couple of weeks ago. It is as big as a Bible. She just turned 10. She just loves books. Always has. Our son is a good reader (capable), but he is just not as much "into" books as my daughter (or my husband and myself for that matter).
I would say that my son has a shorter attention span and had a harder time staying focused (thought he did manage to do it, it was just harder for him) and his favorite part of the lesson was guessing what was going to be in the picture at the conclusion of each day's lesson (you cover it up while they are reading---it helps them learn comprehension skills).
I will also say that my son (now 13) still has a bigger issue with staying focused on tasks like reading, while my daughter does not. She is also considered "gifted", so the age at which she whipped through the book might be a little younger than some kids are ready. The book does recommend they be around age 4, I believe. Can't remember for sure. It's been awhile. But I found the layout of the book worked very well, and the tips and wording for the parents to use when teaching really do work. Be sure to read the introductory section at the front of the book that gives specifics about what NOT to do. :)
P.S. If you haven't gotten the book yet, be sure to check out Amazon, they have gently used copies for $10. And since you don't write in the book, being used doesn't matter unless the kids scribbled in it to the point you just wouldn't want to use it. If you think you'll want to use it more than once (with future children) then I'd buy a new copy so you'll know it will hold up through all the kids. :))