You can use illumos today, right now, and have your ZFS, mdb, DTrace and zones. It really is open source and we're a community using and improving it. For 7 years now already.
As illumos is only the basic building block of the operating system (but unlike linux includes kernel + basic userland) one usually runs one of the distributions:
- SmartOS, developed by Joyent as a cloud hypervisor. Supports zones, KVM and lx-branded linux containers. [1]
- OmniOS CE, a minimal distribution targeted for bare metal server installations [2]
- OpenIndiana, similar to the traditional opensolaris. If you care about GUI this is probably the one. [3]
- Tribblix, modern components with retro style [4]
Sure, the user base is smaller than Linux. But that is also true for FreeBSD which is used by Netflix and Whatsapp. Running a different OS than most other people can give your company an advantage and I know that ZFS and zones have done that for mine.
[1] https://wiki.smartos.org/display/DOC/Download+SmartOS