We've always advised our clients to incorporate breadcrumb trails into their websites (especially large sites). The simple fact is, breadcrumbs give visitors an understanding of where they are within a site's structure, provides an opportunity for them to navigate easily through the category/section which they are currently in and also increases the instances of keyword-rich anchor text featuring on a given page.
Whilst visitor usability and navigation is extremely important, the manner in which search engines treat such links is also important and should not be overlooked. Keyword rich anchor text (which breadcrumbs should nearly always be) improves the ranking potential and relevance of the page which it links to.
This has been the case for some time, but now, there is another reason to have breadcrumb trails featuring on your site. Google has recently introduced breadcrumb trails into it's SERPs, meaning your site now has the potential to have several links featured each time one of your pages appears within the results.
This generally means that visitors have a chance to jump to a more appropriate section of your site, straight from the search engine. What's more, the added links give you greater SERP real estate - which is great! Research suggests that Google is able to put these breadcrumbs into it's results providing you have a breadcrumb trail on your site.
Simply use a separating delimiter such as '>' and you're on your way to having Google add this feature to your listings!