I've just finished optimizing the infrastructure behind WomensForum.com. They have used our Joomla-Varnish package and a few hours of my website optimization consulting time. The amount of money they spent on optimization is comparable to monthly hosting fee for a big website.
While the website was usually fast before optimization, the server couldn't handle traffic spikes. At one point, there were more than 1500 requests per second - that's a lot for Joomla, but not for Varnish. Page load time was consistently low even during traffic spikes. Additionally, we have used the failover capability of Varnish to be less reliable on backend server failures. Speed up your website too.