I joined BALPA and when my company had their backs against the wall and the pilots needed their help, they delegated everything to the company council who are good chaps, but at best only enthusiastic amateurs. Consequently, the pilots didn't get a deal as good as they should have. And in my case, they were argueing that I should be made redundant! Fine loyalty after banking my 1%.
If you're with BA, join. However, other airlines could be better served by BALPA. Perhaps BALPA should be re-named the British Airways Line Pilot's Accociation as that is where their main interest lies. Just have a read of 'the Log' to see this is true.
If I was to work in the UK again, I wouldn't join unless I was in BA. I would go for IPA and TGWU.