I’d say that one good test of how good your product is is how easy it is to use when I’m in a hurry. How fast can I achieve my goal? For example, suppose that your product is a public map that is placed somewhere in an open square plaza, and that my goal is to find the post office, which is somewhere in the square. The faster your product helps me find the post office, the better it is. The good thing about most of these types of maps is that they have a “You are here,” indicator. The bad thing about lots of these maps is that they don’t tell you which way you’re facing as you’re facing the map. I can see where I am on the map. And I can where the post office is. But now what? Now I have to figure out which way I’m facing in order to figure out which way I need to go. If north were always on the top of the map and I were always facing north as I faced the map, then I wouldn’t ever have to rotate the map in my head (map big, brain small) and I am sure that I could find the post office much more quickly.
On the web, it’s definitely worth measuring how long it takes your visitors to do things like create an account, buy a product, or sign up for a newsletter. The longer it takes, the less happy they are. The less happy they are, the less happy you are.