The jbuilder story really has been extraordinary. The initial goal of jbuilder was to facilitate our public release process, but given the speed and enthusiasm with which it's been adopted, it now seems destined now to become the default OCaml build system.
While Simon is right that jbuilder isn't yet fully general, it's a good bet that it's flexible enough for your next project. It's already used for all Jane Street packages, many Mirage and Docker packages, and js_of_ocaml and atdgen, all of which attests to its range. I'd guess jbuilder can handle the needs of 95% of packages.
You can follow along with the adoption by looking at the revdeps in opam. It seems to be growing at a pretty good rate.
My hope is that jbuilder can make a real difference to the OCaml ecosystem, making it way easier for people to get into OCaml.