I've been using nvim exclusively for four months now. Perfectly stable (I think even more so than my vim was, I use lots of crashy plugins) and a perfect drop in, basically nothing changed for me when I renamed nvim to vim on my systems.
I wonder what their roadmap is for a 1.0 release. Is it a specific feature set, or stability in their MessagePack APIs? Is it years off, or just pending some big bugs/refactorings?