We use RethinkDB at CertSimple. It's always been a great DB with safe defaults, excellent documentation and it's always just worked. Trickier ReQL queries are intuitive. It deserved wider support than what it got.
It's depressing that a half-baked constantly problematic effort like MongoDB wins while this gets neglected. Unfortunately we still live in a world where marketing beats merit.
It's always like this. Just look at Windows; if people cared about quality, no one would be complaining about Windows 10 giving them grief because Microsoft would be out of business or relegated to some business software niche, and we'd all be running some kind of Unix variant or maybe some descendant of BeOS or OS/2.