WordPress.org

Does this just work?

Yes.

But how should my varnish configuration file look like?

I have provided a simple VCL that can be used as a reference.

= Does it work for Multi-Site (or WPMU)?

Yes. Activating the plugin site-wide will provide the functionality to all blogs. Configuration can be done on the blogs individually, or can be global. If you want to configure Varnish servers globally, edit wp-config.php and include these lines just before "That's all, stop editing!" message:

global $varnish_servers; $varnish_servers = array('192.168.0.1:80','192.168.0.2:80'); define('VARNISH_SHOWCFG',1);

The varnish servers array will configure multiple servers for sending the purges. If VARNISH_SHOWCFG is defined, configuration will be shown to all users who access the plugin configuration page (but they can't edit it).