For the people wanting to give OCaml 4.10.0beta1 a shot, here is an opam overlay which adds fixes to major packages for them to work with this beta: https://github.com/kit-ty-kate/opam-alpha-repository
To use it, simple call:
$ opam switch 4.10.0+beta1
$ opam repository add alpha git://github.com/kit-ty-kate/opam-alpha-repository.git
Obviously, this repository should not be used in production and probably contains a few bugs, but at least it allows everyone to have almost as many packages available as with OCaml 4.09. Only 60ish packages are still not available, but apart from the notable exception of merlin all the essential packages and dependencies are there.
This work has been part of the release-readyness effort founded by the OCaml Software Foundation as announced here: [ANN] the OCaml Software Foundation
The rest of the effort is going to be put towards having merlin available for OCaml 4.10 and upstreaming all the fixes from opam-alpha-repository (most of them have PRs associated already). I’m hopeful for them be all upstreamed and available before the stable release of OCaml 4.10.