On the HHVM.com blog, there's a post talking about the speed of the HipHop VM (from Facebook) but how it still doesn't support everything built into PHP (and the work being done to bring it up to parity).
HHVM is a highly performant PHP runtime. In fact, it is nearly 40% faster than HPHPc, and only getting faster. [...] Performance is critical, but it isn't everything. In order to gain broader adoption for HHVM, being able to run popular frameworks is a must; in other words, we can have the highest performing PHP runtime, but if doesn't run real-world code without a lot of pain, then it won't be used widely. Understanding this, we are putting serious resources around parity with the PHP runtime.
The post includes a table of features that have been ported and ones currently in the works, based on unit test coverage. They've based it on various well known PHP projects including PHPUnit, Symfony, Laravel, the Facebook SDK and many others. Their initial goal is to allow these frameworks to work 100% of the time inside the HHVM, but to continue the work from there, implementing other PHP features. If you'd like to help out with the process, they also welcome contributions.