SiFive has been late with their U500 or it would exist already. I suspect they're waiting for finalization of the privileged instruction spec. Since they will have PCIe support I'm wondering if you could actually use it with commercial graphics cards and open source drivers. That would put real usable hardware out in the world late this year.

There are other companies on the verge of releasing hardware implementations. But in every case, I would wait for the final specs before jumping on any particular hardware even if it shipped now. Other than the micro controllers of course.