Had a similar experience deploying HHVM at a previous company, I wrote up a blog post of the issues we ran into / how we worked around them[0]. One thing the dailymotion blog omits is hacklang which has additional features like lambdas, async support (though PHP 7 will soon?), strict typing, collections, generics and more. That said, if you're just trying to squeeze more out of an existing codebase, then PHP7 wins hands down.