As a Meta employee for almost 4 years what I will say is I was skeptical at first coming from git, but the sapling system works very well in practice in my experience. I still use git for everything outside of work, but I may consider sapling now.
I'm ex-Meta and now at Google and while they have 'hg' as a wrapper around their fig system, it's lacking so many of the Sapling features I am sad and frustrated occasionally.
