I don’t know when all of the multicore efforts might land in the main OCaml distribution, but my experience with the remote that’s been baselined on 4.06.1 has been pretty positive so far.
One thing I noticed today is that PR#1003 to bring the C interface changes into mainline seems a bit stalled. Apparently, the documentation needs updating (and I might be able to help out with that) and there were also apparently some efforts underway that might make some of the breaking changes unnecessary (which I’d be curious to see an update here about that).
Certainly it would be nice to have the OCAML_API_VERSION macro available so that we can begin preparing external projects for any breaking changes in the C interface going forward. I’d be thrilled to have just that in the OCaml 4.07 release.