Having recently acquired a shiny new (and very expensive) eyetracker we were keen to understand how best to use it. So we sent one of our staff off on a suitable training course. Our chap came back with lots of good new approaches and techniques but what surprised me was that no mention had been made of what I see as the single biggest benefit.
We adopt an approach to usability testing of websites that starts from the premise that a client understanding of the usability issues and then developing solutions that fit their context is absolutely vital. You can come up with all the recommendations you like but if the client doesn't think these address his problems, or they can't implement the solution for some reason then you are wasting your breath - or your report paper!
We, therefore, get clients actively involved in observing the user testing and 'co-create' the solutions by discussing the issues identified by the testing. What the eye tracking does, because clients can watch it in real time, is show a lot of things we would normally have to explain (e.g. users spend time looking first in the centre of a navigation page not at your nice navigation bar on the left where they often need to be for their goal!)
For most of our clients the information this provides is a revelation. We know that users won't read anything but look at headings, bullets and links before they find the actual content they want, but that doesn't stop the vast majority of websites filling their navigation pages with content irrelevant to most users' goals. This simply slows them down finding what they want.
Watching the eye track as the tester uses the site shows the client all this in horrifying explicit detail - and I don't have to say a thing!