HelenaGwyn our cache feature is very basic - it sets a cache expiry header for browser-side caching. It's mostly useful only for purely static assets.
Most cache plugins for Wordpress go beyond browser-side caching, for example they can pre-render pages to HTML and serve the HTML instead of spinning up PHP on every request, or they can do object caching in a memcached instance, etc.
IMO the best approach would be to use both, for example you can serve your WP static assets from a symlink app with our caching enabled and use a caching plugin to do server-side caching where appropriate for dynamic pages.