Measuring how fast your site really is

Google Webmaster Tools recently added a new performance section that shows the average load time for your site as reported by Google Toolbar users. This is a great addition of really unique data. Look for it in the labs section.

It's hard to measure this any other way since it's hard to predict how much bandwidth your users have, how fast DNS resolves, whether their browser cache has any content from your site already, etc etc etc.