My dream job. (No, seriously, it was my dream since about maybe two years ago, before I sent the first PR.)
In terms of what it's actually like, our work is generally pretty visible. Lately I've been working on Create React App because a lot of people in the community are frustrated by complicated tooling. I also send some PRs with improvements and bugfixes to React. For example I added a Chrome Timeline integration to the performance tooling that will be out in 15.4.0.
I live in London so I talk to the team once a week via a conference call to sync up on tasks and priorities. I try to publish our meeting notes on https://github.com/reactjs/core-notes.