I did some tests on my server (i don't have APC or similar installed), only W3 Total Cache plugin caching on disk (pages, objects and database).

The weird part is the page generation time, plugin needs more time to generate a page when i have object cache enabled. If i disable object cache plugin need less time to generate the page.

The plugin needs more time in page generation process, but perhaps is using less resources... i´m not sure. But i´m starting to have doubts about how beneficial is have object cache enabled on disk.