It's beyond experiments now and there are clear uses for such things. For example, a recent post describes using MirageOS to build a firewall VM [1]. There are also network security/intrusion-detection systems [2].
There are a number of ways to build unikernels and they're not all OCaml (which is just MirageOS) -- http://unikernel.org is trying to bring these together.
[1] http://roscidus.com/blog/blog/2016/01/01/a-unikernel-firewal...