I thought it was useful tbh, though as with her other books I'm not someone who takes it literally - I just take the gist and sees what works.
Her list of what they should be able to do as preparation was very useful - I learnt the hard way that they need to be able to manage to pull up and down pants and trews. Also I followed her advice on tracking accidents etc and when to give up.
Training DS1 took 2 goes: the first when he was 2y 3m (and was on "my timetable" which lasted a week and then I quit. The second was at 2y 6m (when he had fulfilled the GF pre-requirements!), and we were done within 10 days or so.