I've been a member for about 17 years at various grades, and have a great deal of time for RAeS, in terms of what it gives you - library services, networking, conference attendance, local branch meetings, all sorts. Those have been invaluable in my career to date, although I have to confess I've never actually used the careers service.
I'd suggest joining as an affiliate (cheap and painless) right now to get access to the services, and also go along to the local branch meetings to see what people say.
There are a great many senior pilots in the society, but I honestly have no idea whether they've found it helping their careers or not.
G