I, in common with Shunter and about 25 others signed up with CATS for CPL/IR/ATPL groundschool distance learning , ( in my case ICAO/PPL/IR conversion.)
I found the course excellent: the materials both online and printed as well as question banks were very relevant and I got the required 7 exams (for conversion) out of the way in two CAA sittings with average marks of 94%. The course started in March and six of us had completed the groundschool by August and several well into the flying training. (I start next month)
Now I'm very happy with distance learning and the HOT laid on (still ongoing) extraordinary weekend ground school sessions for clarification of difficult stuff.
I don't think I would have got such good marks ,and virtually everybody (only a couple of single paper fails) got first time passes, without a good teaching materials /groundschool, with small classes and personal attention: Being totally mathslexic I even got a one to one on the operation of the scientific calculator.
However: if you aren't disciplined enough to use distance learning and need spoon feeding then perhaps CATS might not be for you.
I personally found it very good.
Cusco