I've resisted joining a professional society ever since I was an undergraduate, due to a mixture of scepticism about its value for money and a desire to get by on my own merits rather than (effectively) buy any recognition, so this is in some ways a change of heart for me.
I think the major difference is that with a reasonable amount of professional experience now under my belt, if I go back on the job-market then I expect that will outweigh membership of a professional organisation by quite a way. As a result, I no longer feel that I'm trying to put a thumb on the scales by joining. Of course that's not really what groups like the BCS and the IEEE are all about, I don't expect membership would really have made any difference to my employment prospects at all, and it's a daft thing to worry about anyway but all the same I feel more comfortable joining now than I could ever have done then.
It will be interesting to see whether it feels like money well spent. There are some obvious benefits - receiving a magazine and discounts on some commercial software - but I expect the biggest benefit will simply be in encouraging me to do something about structuring my professional development.
The letters are MBCS CITP (Member of the British Computer Society - Chartered IT Professional).
Update: a congratulatory e-mail has just gone out at work which makes me sound like a god. How embarassing. Still, it all helps build the image of the department.