Michel Baudin has an interesting blog post (http://michelbaudin.com/2012/07/19/iso-9001-conspicuous-by-its-absence-quality-digest/) on ISO 9001 and the before-and-after evidence regarding its benefits and effectiveness.
I agree with Michel’s point about the problem-prevention aspect of ISO 9001. I’d also add that an overlooked strength of ISO 9001 is the process approach to managing a business system. This aspect of the standard is often overlooked in the rush to get certified and never really gets fully implemented in the way it should within many firms, because the focus shifts to the clauses of the standard and meeting their requirements.
Few firms really understand and manage their businesses as systems of interconnected and interdependent processes. I have always felt that the real strength of the ISO 9000 approach, particularly in the latest (2008) version of the standard, is the process approach and the need for adopting firms to focus on understanding and meeting customer requirements with a well-managed and effective process system.