The vkernel has been broken the last few weeks but Antonio and I have been working on fixes, and I've improved the performance (on master) as well. Its still being tested but I think it should be stable in the next day or two.
In terms of usability, it depends on the use-case. vkernels are very good at capping real system resources, but have high thread switch and syscall latencies relative to a real kernel. Jails are high-performance but can't manage real-system resources very well. -Matt
