It depends on his learning style. Some young people love the CGP style, others find it a bit 'busy' in layout.
DS1 likes a revision guide to be laid out so that a topic is on one page or double page spread.
This is what we have bought so far:
Sciences (CIE iGCSE): we have bought textbooks, as none of the mainstream revision guides were specific to the board he is doing. Each topic is on a double-page spread and there are some questions from past papers included. He is finding the Physics and Chemistry quite tricky so has been doing extra study on these at weekends.
Maths (Edexcel): again, we bought a textbook (Rayner) as it follows the same scheme of work that they are doing at school. He revises Maths by doing practice questions and then marking them to identify areas of weakness.
German (AQA): we bought the Pearson revision guide & workbook, as you can also access audio files for the listening part. Disclaimer: he hasn't yet opened these, despite having a controlled assessment coming up.
History (OCR): this is his favourite subject, and he wasn't happy with any of the revision guides we looked at. He is making his own revision notes using his textbook, BBC bitesize and the why/what/how questions in the course specification.
Latin (WJEC): there isn't a revision guide, but he does practise using the Cambridge Latin website.
Classical Civilisation (OCR): again, there isn't a revision guide, but this is another of his favourite subjects. He is making his own revision notes.
English Literature (AQA): I have bought him the York Notes for the texts he is doing. He has looked at them once or twice, and also looked at BBC bitesize.