I think that every pilot (or engineer for that matter) should join at-least one significant representative organisation - we very much need them.
Which organisation depends very much upon where you are and what you fly.
For me, in the UK, I belong to two national representative organisations: LAA and BMAA. Both do a superb job of representing their members interests and organising things I want to be a part of.
AOPA UK is small, heavily influenced by its corporate members, and for many of us a lot less relevant than the big three: LAA, BMAA and BGA.
AOPA US, if I lived there, I would certainly join - it is extremely impressive.
Regarding Belgium - I really don't know, but it can't be that hard to look and see what impact the one or more organisations present there - then make your own mind up.
AOPA US does some great magazines and may well be worth joining for that alone, but I can't see they're going to help you much as a Belgian pilot.
If there is no equivalent in Belgium, you could do worse as a new microlight owner than join the BMAA. There is much information and advice available through them, even if you don't fly a British registered microlight.
G