Jan: yes you should join
AOPA USA. I have been a member since 1984 and do not regret it. If nothing else, the
AOPA Pilot magazine is good enough a reason.
Also, if you fly in Europe, you should consider joining
PPL/IR Europe. PPL/IR is a great organization for private IFR flying,
tries helping European based N-reg GA aircraft, provides excellent resources, a color newsletter and web site (including forums).
And,
EAA - The Spirit of Aviation - Oshkosh, WI of course
(and participate at the AirVenture Convention taking place there at the end of every July).
You will not regret attending the
AirVenture convention, that's for sure!
All three are well worth joining.
Good Luck! Happy & Safe Flying,