I belong to too many! A combination of hobby-aviation, professional-aviation, and professional-engineering.
All have substantial value in their own ways, or I'd not keep paying the fees.
I don't belong to GAPAN (or it's sister the Worshipful Company of Engineers), although I've thought hard about it - at the end of the day, it would be just one association too many. But I know that many people get much from it.
Ultimately you have to decide what suits your aviation interests - but do belong to at-least one organisation, because in this world, there is much reason to belong.
G