Just a heads up, and the post notes this, that this way of timing is already well into the page load sequence: the server has received the request, built the HTML and has started sending it, and the browser is already parsing it.

Also, be careful with when different browsers fire the onLoad event. For example, Safari 3: http://www.howtocreate.co.uk/safaribenchmarks.html

Overall though, a cool hack even with the caveats above as it could give you actionable data.

Another good source of loading time metrics is Google Webmaster Tools. They have two metrics, one in the download time in the Crawl Stats section the Site Performance metric in the Labs section. Combined, they give you a really good insight into how your website is doing over time. A good game to play is spot the (very clear) inverse relationship between the number of pages crawled vs download time.