User Interface Engineering has posted an article about breadcrumbs, and why they feel that breadcrumbs are a design cop-out. It is a good read, especially for people using SharePoint for external facing sites.
Personally, I only utilize page breadcrumbs when working with administering lists and settings. The breadcrumb provides a quick way back to list screens and other setting screens where otherwise no navigation is provided. For that purpose I think they can be useful. The article brings up some valid points on whether or not to use breadcrumbs at all.