This is so important. We do routine usability tests all the time simply because it's hard for us to see the detail in our own product.
A user can stumble across the most subtle things that take two seconds to fix that otherwise we would have never noticed and we'd keep losing customers because of it.
More people should do usability tests. They're easy (see Steve Krug's books on the subject), and very eye opening.
If I had to pick between programmer-written tests and usability tests (if! you don't, of course) I'd pick usability tests, because programmatic software can't tell you if you're building the wrong product.
A user can stumble across the most subtle things that take two seconds to fix that otherwise we would have never noticed and we'd keep losing customers because of it.
+1 for attention to detail.