It depends what you are looking for.
DD is dyslexic with very slow processing speeds, and a consistent ability to underperform in exams. Our experience with English iGCSE was very positive. First she gained confidence that she was alright and that she had been well taught, something that is not obvious if you trail at the bottom at a selective secondary. And then a much clearer idea of what examiners would expect to see in order for her to get top marks. (Oddly perhaps, school teachers often try to teach the subject rather than try to drum mark schemes into pupils 
)
It worked and she walked away with a completely unexpected A* in Eng Lang, which has been really useful for medical school applications.
Interestingly there were five boys in the group of eight, all from the same private school. They apparently were way behind and simply had not been taught what they needed to know. I assume there had been a level of parental panic and JC was a quick way of accessing reputable tutoring.
DD also did a couple of AS courses. I'm not sure if she needed them, but her dyslexia means she learns much better in a classroom setting than from books, so they provided a level of reassurance, plus provided a chance to get out of the house. She said the teachers were good and well organised, and usefully having someone else explain a physics concept she had found difficult led to a lightbulb moment when she suddenly got it. She enjoyed them, and a run through the syllabus together with some coaching on exam technique wont have done any harm, and possibly she gained the few extra marks that took her A2 predictions up a grade. But I doubt we would have bothered if this sort of revision approach did not suit her.