From unixy blog, where they have upgraded from Varnish 2.X to 3.0 and after running some tests.
They have come to this conclusions:
It's important to note that the resources allocated to both experiments are the same and the goal is to stress test 2.1.4 and 3.0.0. Release 3.0.0 is over 600 requests per second or 10% faster than 2.1.4, which is great! Congratulations to the Varnish Cache dev team for job well done :)
Well, all I have to add to this, is that Varnish is really a great software I’ve tested it, and I can see a lot of improvement when using it.
I'm sure that digg, Slashdot or Hacker News site can not take your site down if you are using varnish and serving serving static pages.
Of course if you need to use too much plugins that generate too many SQL queries, varnish may not be your best bet, or you will have to add memcached to it.
Memcached increases the performance and scalability of dynamic MySQL-driven websites by caching data and objects in memory in order to minimize database load. The biggest names on the web, like YouTube, Facebook, Fotolog and Wikipedia deploy Memcached and MySQL to satisfy the demands of millions of users and billions of page views every month.