From what I can gather Varnish doesn’t do much caching. It will I think serve stuff from the cache if the backend server doesn’t respond quickly enough which explains why I can use ab to fire 100 parallel requests at the server now. It also treats images and other static content as cacheable which helps too but because of the cache busting in WordPress I don’t think they’re cached very often.
I didn’t do anything special to make it multisite aware however. The site feels faster, even as a logged in user so I’m happy. Then again my traffic has tanked in the last 6 months so it’s not particularly busy any more!