I'm hardly a great judge, having just started with the language myself, but after years of high level dev (in language terms, not skill) using Perl, Python and PHP for web, I find that C can take you so close to the metal on the platform that you're working on, that it becomes less of a language issue, and more of an architecture / platform / OS issue.
C has few keywords, the same-ish loop structure as everything else you've used, so "learning" C is pretty easy. Learning "how" to use C could take a lifetime. Just poke through some of the docs on a friendly FreeBSD server, or look at the source code and see what can be achieved. It's mind blowing.