I've managed a lot of content — brochures, landing pages, one-pagers, videos, web sites, email, case studies, press releases, infographics, ebooks, social content, articles, blog posts, plans, briefs, styles guides, brand and identity guidelines. I've done it all. And I've written a lot of it.
Content is a success for me when it solves the problem it set out to solve (IOW meets the goals outlined in the brief and meets the metrics it set out to achieve).
It's even better when someone tells me they learned something from it or were inspired by it.