Only "in additional to npm", and then really only once yarn is at parity. Right now yarn still has issues installing certain packages, and there is no yarn equivalent to "npm run ..." which is a pretty significant hold-up (projects that rely on npm scripts rather than make files or some other OS-encumbered approach cannot switch over at the moment).
It's in the works and slated for a later release, but makes it such that yarn is not a universally viable replacement for npm yet.