An attempt at a balanced view
I used CATS for my CPL(A) distance learning groundschool and tend to side with the critics.
The organisation was very shambolic - very hard to get joining instructions, details of what would be delivered then, or sometimes the right manuals when I needed them. Several time I and others had a strong whinge about that, and not much was ever done about it.
Far too much of the classroom time was just doing practice tests - that could be done in time I'm not paying for! The teaching, when the right teacher turned up, and actually decided to teach stuff rather than set yet another practice test, was pretty good. I absolutely agree with the praise for Mike Burton, but there weren't actually any bad teachers - maybe some whose personalities I found easier to get on with than others (hearing for the fifth time about somebody's self funded 737 type rating got a little tedious).
The notes are extremely good, and the way in which they structurally take you through the material, and test you at each point, then take you through to summary tests, is superb. You could make a criticism that the notes can be a little too long and detailed for just passing the exams sometimes, but as somebody who tends to be interested in technical stuff, this wasn't too much of a problem.
Ultimately, I passed everything first time and with a good average of 86%. This was partly to CATS credit - particularly their excellent notes, and partly due to also subscribing to the Bristol questions databank which was incredibly helpful, and also to buying a copy of Phil Croucher's "JAR Professional Pilot Studies" which was very useful in cutting my reading down sometimes to just what I really needed.
Of course, whatever course you do, you need to work hard and be self managed and motivated - I can't imagine that's any different for CATS or anybody else. But, it did seem to be particularly essential there.
Ultimately they were "okay", but would be a whole lot better if they were better organised in terms of lecture planning, communication with students, and general admin. I'd give them about 6/10, and it wouldn't take much management reorganisation to get that up to an 8/10 - although there seems little will to go that extra mile; probably they realise that most people (like me) go to them because of the price and people will keep doing that regardless.
P