My feeling on this subject is the best approach is to inject javascript into the page and have that report back metrics on the user experience. There are various approaches to this but they basically resolve down to either rolling your own code (more work but ultimately better results) or using a third party offering. New Relic have a pretty nice solution.
The problem with this approach is you still need something to exercise the browser in order for the javascript to get fired - it is hard to automate. That's the gotcha but other than that you have a reliable, consistent, maintenance-free method for recording and logging the user experience.