A year ago PHP7 launched. Those already using HHVM ran a bunch of benchmarks and came away with the conclusion that HHVM is much faster and is indeed the secret sauce of their superior pricey hosting. While scouting for hosting for a very high traffic site, I heard this from WPE, from Pressidium and from Kinsta. Out of the three principal advocates of HHVM, Kinsta's offer was the only one which made any kind of financial sense.
Since then, we've been using HHVM for that project with no end of grief in surprise collapses in the middle of the night or the middle of the weekend. Are the instability and incompatibilities worth it? For a smaller web publisher, certainly not. For a middle-sized web publisher (think top 2000 website in the US), probably not. For a very large sized web publisher (think Facebook or another top 100 website in the US) quite possibly. Here's why.