Thanks for the heads up on the EuSec, I will look into this for next year.
As for the BCS events, I used to be a member about 5 years ago, but never renewed as I really didnt get much benefit, perhaps now is a time to reconsider?
Depends on your local branch I'd guess. I've been a member for around 4 years and have been to around a dozen events. Just depends on your interests and who they've got to speak. Had a really interesting debate when the head of BCS did a talk on professionalism in the industry and basically got attacked very lively debate.
From the perspective of specialist groups, for me they help me keep semi-upto date but the events are all London based (I'm
definitely North of the M25

) so I don't have a great opportunity to attend so can't comment.
The forums (especially security) are, imo, fairly weak. Anything that gets discussed I've usually known about for a while and/or most comments are a re-hash of either the Daily Mail or /. But it's been a while since I've looked, they may have improved recently.
In terms of return on investment, the BCS newsletters/emails usually have a good article or two and I get a free copy of
Computing every week. The discounts and special offers have never really helped me, but the new online library could be interesting as it expands.
Biggest benefit I've gained from it is the CV aspect, one more plus I've got over another applicant.
Your mileage may vary, but I'd at least suggest keeping an eye on the websites for your local branches to see if there are any interesting talks. Most are open to non-members anyway.