I used it but didn't follow it to the letter.
I found it incredibly helpful in structuring the day of my DC1 because he seemed to be taking what felt like a hundred tiny feeds and catnaps during the day and no one could tell me what he should be having "whatever he wants, dear." GF basically said that most babies sleep for X number of hours, in this sort of pattern, try to feed for as long as possible four + times during the day and do two naps and they'll do a longer sleep at night. It was fab, and DS1 just didn't cry unless hurt or unwell, he would wake for food when I expected him to so I was already ready to feed him. It was extremely harmonious. Then I had DC2 eighteen months later and put her on the same routine; I'd have 2-3 hours to myself in the middle of every day while they both napped - saved my sanity!
The one thing I didn't do was wake the baby for a feed at 10.30pm. I usually go to bed around 10. Tried it once - made myself stay up and ended up with a wide awake baby who thought he was up for the day at 11pm. Bugger that! So what I did was let them wake me for a night feed whenever they wanted, usually about 4am - much better!
My third DC was born a few years later and I went back to the book, and was surprised at how many corners I had cut. But still - I think it's great!
I am always surprised on these threads at the things that are attributed to her. I don't ever remember reading that babies "just cry for attention" written in that way (I mean, they do cry for attention, that's the point of crying!, but I don't think the book is anything but sympathetic to babies, and the idea that babies should settle themselves to sleep is a good one, we all need to be able to settle ourselves to sleep.)