I joined at 19 as a student and was its youngest Fellow at 35, still in over a decade later.
Very very useful to me in terms of professional contacts, library facilities, free meeting rooms in London, access to the top people in our profession. I've addressed the House of Commons (well, one of the select committees) as a representative of the society on a subject that mattered a lot to me - that sort of influence is exceptionally valuable.
It is however as useful as you're prepared to put the effort into making it. If you just think you can pay the money and the benefits come - you would be wrong. You need to attend the occasional conference, engage with the odd committee, be prepared to roll your sleeves up and help organise things from time to time.
I agree totally with DaveReid - go for one of the earned membership grades, not just Affiliate which is (with all due respect to those brethren) basically the spotters grade. MRAes cuts the mustard, FRAeS even more so. AMRAeS is on the way and still worth pursuing.
The magazine is good too.
Sadly, it's also quite expensive - especially at the higher grades and/or with professional engineering accreditation. But I'd still not leave if I can possibly continue to afford the fees.
G
N.B. Allan - the main part of the Library moved to Farnborough several years ago.