It all depends on you. If you're a motivated individual, with the discipline to knuckle down and get on with it, CATS are just fine. Don't EVER expect to have groundschool save the day if you haven't put the work in. Read the books, do the progress tests, hammer the question banks. You'll soon know yourself if you're ready for the exams. The groundschool is simply chance to ask questions, clear up any concepts on which you're unsure, and get exam forms signed. I passed my exams with no problems whatsoever with CATS. If you need your hand holding from start to finish and can't force yourself to do the work (plenty of 'em on here), then pay twice the price and go elsewhere.
Bristol have an excellent reputation, but they are a lot more expensive.
Some of the stuff on here is true, some of it is wildly exaggerated. If you've got a brain in your head and want access to the exams (ie. signed forms) with the bear minimum of obligation in terms of sitting in a classroom, CATS works. If you place value on things such as plush classrooms and whether the coffee machine is maintaining an exact coffee/milk ratio of 12:1 go elsewhere.
A standard day at CATS involves knocking out a test paper first thing, then going through the answers and expanding on questions which people are having issues with. Little things... like how to do QNH/QFF questions, handy formulas etc... Turn up not knowing how to deal with lambert charts, and what the difference between a great circle and a rhumb line is and you're your own worst enemy.