Make Network Fast with Cache - But Which One?

Morning everyone...

I've been reading about the wide range of cache plugins for multisite and am more confused than before I started. I ended up with many questions...

Do I need one? That's the first question. At what point (amount of visitors) is it going to make a big difference to the load times?

I read on one website (not sure which one, now) that it is best to 'complete the site before installing'. That confused me. Surely, by the very nature of WP or any CMS is that the site is never actually 'finished' and new content (pages and posts) is always being added.

Can they (or any particular one) be as simple as "set it and forget it"?

I can't think of a better place to ask for a bit of feedback about "Choosing The Right Cache Plugin for Wordpress MultiSite" than right here...

Opinions? Thoughts? Experiences?