Michael Angeles has written a truly excellent blog entry on using logs to determine the effectiveness of navigation. This is a must-see entry due to the concrete research, and fascinating results. To quote:
I need some way of exploring alternatives, but I don’t want to change the site too drastically without having some justification for doing so, so about a month ago I asked our systems administrator to come up with ways to track where links are being clicked on the page. The first month of data has arrived and I have some stats to show where people are clicking to get around the site. The wireframe and the charts below show what we’re seeing.[Column Two]