In his latest post to the Liip blog Christian Stocker points out that the New Relic extension for HHVM has been updated for the latest versions of HHVM to work a bit more seamlessly.
Since HHVM 3.4 it's theoretically possible to have your own external profiler for function level profiling (like xhprof or xdebug) without having to recompile HHVM itself. Unfortunately it wasn't perfect (or I couldn't make it running), but there's a patch in the master branch now (the upcoming 3.6), which seems to solve that problem. So I worked a little bit on my extension in the last few days and I adjusted a lot of things and improved some other stuff.
He talks about the improvements New Relic has made on their functionality and some slowness that still exists in the "hotprofiler". He points out, however, that if you just want overall statistics and not specific, method level ones, you don't really even need to use it. He offers a word of caution when using his extension and when it may fall back to "userland level profiling" instead.