CGP are good for most subjects, such as Science and parts of English, but I found their SHP History book to be a little too factual. I had used a bit of it for my resit, but found the official revision book so much better that I got an A* in it from a D.
But still, I'm not very good at Science, and I did manage to pass my Science GCSEs pretty much only using CGP. They may be a bit silly in some places, apparently one book has a page which was only about cows for like a French exam or something, but it seems a lot better than some of the extremely dry official books which just pretty much say, "Page, facts, question, read it, bye."