John
I did ATPL DL with them finishing end of 2006. You are correct, the books are not great, certainly nowhere near the standard of other manuals available and often merely a duplication of notes given to integrated students with a learning 'timetable' bolted on. I recall reading in instruments "remember + My Tummy Churns -" which had no further explanation but once I clarified what it meant was one of the pieces of info I used most often in the exams (relationship between airspeeds), plus remember one laughable description of radio nav aids saying 'red' 'blue' etc but manuals were all in b&w! (Subsequently found out the originals held by Cabair were in colour: let you draw your own conclusions from that). I used the Oxford and Jepp manuals avail on ebay to augment the Cabair notes. All that said:
a, the CGI is trying to improve the manuals and intro some CBT - and the manuals were getting better, heck one of them (Air Law) even had some colour pages!
b, you 'tune' into the manuals after a while and learn to interpret them.
c, the residential instruction is excellent and you will learn more in those weeks than by any amount of reading.
d, Cabair split the exams into 4 manageable chunks and so it is easier to cope with the course on top of a fulltime job and family.
e, they will teach you the syllabus not just the current exam material and so you will be able to work out what you do not recognise from the BGS database. I got mid 90s average (infact over 90% in every exam except one which I got 89) and first time pass in every subject.
If you are having doubts maybe you could ask to see the current manuals and sit in on a lesson? If you can cope with the fact that you might have to augment some of the muddier manuals, and remember that the onus is on you to contact them if you have a problem - they are not psychic - then you will be fine.