Support for Varnish purging is one of a couple of reasons why I switched from WP Super Cache to W3 Total Cache (sorry!!). Everything’s super quick, Varnish serves the static files much quicker than Apache, and the number of concurrent users Varnish can handle is positively obscene. With WordPress now purging Varnish when content is updated, I’ve barely had any situations where Varnish is serving out-of-date content, too.
Varnish is genuinely amazing. Supremely capable, and with a learning curve that’s essentially flat. You can get it up and running in about an hour, and can get a pretty awesome hit rate in an afternoon — it’s lovely.