We use a detailed step-by-step user-interface regression test for our commercial web application. It has a "backbone" test for the most used / most important parts of the system, with optional tests for specific areas of functionality. Using this plan has definitely helped us ensure high quality software.
But, having very specific tests can be counter-productive. The tester concentrates on following the test and will completely miss usability issues, or not notice fairly obvious problems such as the bottom part of a page that is missing.
By contrast, some of the best UI testing happens when building a demo of a new feature. I often do my own best testing by pretending to demonstrate the system to an imaginary prospect. Yet when I tell the testers, "Just demonstrate the system to yourself" they don't cover nearly as much functionality as they do with a detailed point-by-point test.
I'm repeatedly asked to provide more and more detail in the test plan so that a new untrained tester can test with it without asking any questions. Yet details seem to be counter-productive. How much detail do you put in a regression test to make it effective? How do you write tests that make the tester focus more on the system than on checking off items on the test?