Technically we use Newtonsoft and Jil, Jil replacing Newtonsoft as we become increasingly confident in it.
I wouldn't suggest anyone use Jil in a production role unless you're at Stack Overflow. It's too untested at the moment, and the typical person can't get me on the horn to fix whatever just broke.
You wouldn't right now (Kevin doesn't recommend it). But in the end it you'll want to use it if JSON serialization is a performance bottleneck for you.
I wouldn't suggest anyone use Jil in a production role unless you're at Stack Overflow. It's too untested at the moment, and the typical person can't get me on the horn to fix whatever just broke.