A lot of their good work is necessarily behind the scenes.
Personally they have given good advice about employment contracts, got a bullying manager off my back and successfully resolved a legal issue in my favour.
In my experience their judgement is pretty good and they have a lot of experience to draw on. Now, the tricky bit is that they won't necessarily leap out and defend you. You have to push and prod them sometimes. Equally, they will only take on a legal case if they thing you have a greater than equal chance of winning. This is why you might need some legal insurance to completely cover you.
However, there are some situations where BALPA really are the only body that can deal effectivley for you.
For whatever reason there seem to be more tricky issues surrounding pilot employment these days. Don't wait until you have a problem to join.
Just think, your Captain taxis on to stand in driving rain and you manage to hit a GPU. You are just about to get your command and the company wants to know why you didn't tell the Captain to park short . . .
It happens!