I did CATS distance learning plus the mandatory couple of weeks in the classroom. I did CPL rather than ATPL writtens, but got first time passes in everything, as did everybody I knew doing ATPLs alongside me from CATS.
I did buy Phil Croucher's book, which I found a useful simplified reference alongside the CATS material and still sits on my desk at work, and I did spend a bit of money on a short subscription to the Bristol Groundschool question bank for last minute practice for the exams.
But, basically, yes, it was good enough. The school themselves were often a bit shambolic, but I've absolutely no complaints about the written material.
My study strategy was broadly similar to any other knowledge based exams I've ever done. Read - precis, re-write my own notes a few times with reference to the books, re-write my notes without reference to the books to make sure I could do it from memory, practice questions, back to the books to correct mistakes, more practice questions, take the exam.
This was 6-7 years ago, but presumably the material is no less good now. I'm sure that good learning practices haven't changed.
G