Field Report: Lessons Learned from 1-on-1 Usability Smackdown
blog.skritter.com
blog.skritter.com
Not enough people do it, and those that do are always surprised at the results. You can't second-guess users, they are unpredictable. Well not entirely, but the factors to consider are too numerous and complex to predict reliably.
I now rarely accept arguments for change on the basis of "usability" unless it has actually been visibly demonstrated by real users, rather than hypothetical textbook ones. Kudos to Skritter.
When switching contexts though, nothing will prepare you for the myriad of ways people can and will use a product, and tests become a necessity again.