Ailsa, workplaces where the majority of staff belong to a union (and therefore may go on strike) are more highly-paid than non-unionised workplaces.* So union activity works, obviously. You could extend that to mean striking works, if it gets to the point where an employer is being unreasonable and inflexible.
I'm not involved in this one directly, but personally would find it very hard to cross a picket-line. The whole point of unions is that together people are strong and can stand up for themselves against those in power and all that... although my sister, who has the same attitude, did cross a Unison picket line once (not her union and ethical objections to leaving vulnerable people without care).
*In industries with unions, obviously. Don't think it applies to merchant bankers, for instance....