I removed my DS from RE after reviewing the syllabus.
A child of a.n.other religion was allowed to do private study in the library, however my DS had to remain in class albeit with other work to do (a syllabus I created for him and even provided exams for at the same time as others were doing RE)
I found the attitude towards me was very condescending - a 'well if you don't believe in a religion, why do you care what he's learning' type of attitude. Initially I was told since he was not being withdrawn because he was of a.n.other religion, that I was not allowed to withdraw him. Which is rubbish.
The mechanics of the opting out were, letter to school, meeting with HOD & RE teacher were the syllabus was discussed, I received a copy of it to look over in more depth and was verbally told it would not be possible to withdraw him unless it was because he was of a different faith.
Then a further letter to school stating the appropriate, correct, guidance about the right to withdrawal and that I was exercising that right and formally withdrawing DS (and would provide an alternative syllabus for him myself)
I did not have to review this every year with them.
I suspect, especially after the DENI link, that some of us are posting from NI were RE, and how it is taught, can vary widely from what is taught elsewhere in the UK.
A generalised RE syllabus looking at different religious beliefs and ethics etc would be fine imo, a syllabus very much based in one religion, or rather often here, in one branch of one religion, is not fine for me, or my DS.
Do ask for the syllabus, do have a chat with the teacher, then decide for yourself, not only if you are happy with it, but also what alternative you are going to provide (if any is asked of you).
I don't regret taking him out of RE for a second.