I am not a BALPA member, and for the following reasons.
The last 2 companies that I have worked for have paid less than token lip service to allowing BALPA to 'work with them' in respect of pay deals and so on. When it came to redundancies earlier this year BALPA were allowed to negotiate, but eventually the company went it's own way, and ignored BALPA. At the previous company always allowed negotiation, and always went their own way. The CC eventually resigned en masse because they had not received proper support from BALPA. It took BALPA 6 months to get a new CC, and during that time there was no communication from BALPA to the members.
There are larger issues where I think that BALPA have done pilots a disservice too. The biggest one I believe is that of pilots paying airlines for type ratings. As far as I know, and whilst I was a member I asked the questions often, BALPA did not oppose this policy at any stage. It is now nearly de rigeur for UK airlines.
BALPA also failed to negotiate rostering agreements in both companies, despite the regular lip service from the company that they wanted one, and the continual dithering, and non achievement of any such document which resulted in ultimate flexibility for the company, and none for the pilot.
SAS makes a very good point about pilots as a group. In the crew room, and on the flight deck we talk alot of bravado about not going into discretion, and not allowing rostering to take the p*ss in order to run an unworkable schedule. When the phone rings, and crewing ask you to work a day off there is always someone who is willing to take the £300 for an FO, and £500 for a captain. Everyone thinks that if they make some sort of stand they will put their career on the line. For those reasons I chose to withdraw my support of BALPA, and whenever I had any questions about what was being done I referred directly to the Base Captain, and the Chief Pilot.
I don't think BALPA had many teeth, at least at the companies that I worked for, so I don't really believe that I received much in the way of benefit that I hadn't paid for, and I certainly didn't lose sleep over any that I might have received.
There is a point about BALPA that is very relevant, and has been hinted at today. The CC are nothing without the membership. They cannot represent the views of the pilots if the pilots don't make their views known, and relay them to their CC. The membership too often expects the CC to read their minds, and then get up in arms when the CC don't do what they member wanted.