For sure, don't use the Maths in Focus textbook to learn the entire course from, Cambridge is definitely superior in that regard, but if you find yourself looking for more questions on a specific topic, which can be hard to find from past-papers, it has some value. I utilised the Cambridge textbook for all my internal assessments, so by the time that I got the HSC I could essentially remember most of the review and chapter answers and since I had MiF, used it occasionally for both Extension and Advanced (particularly advanced). Unfortunately, I did end up relying too much on past and trial papers from other schools - most of the latter did not quite accurately represent the HSC.