Me! Yeah, it's not 100% compatible, so my app runs all it's tests on HHVM to make sure I haven't run into any edge cases, which hasn't been an issue. I get more mileage out of my smallish VPS from it, with no effort on my part other than knowing what frameworks don't work (very very few now) and testing before deploying to staging (as you should be anyway).
I'm pretty happy with it. Is it worth it? Well, if you've got a test suite, run it through HHVM to see if you'll have any issues. If the answer is no, then why turn down free performance :)
I run HHVM on a website. I started in June (back then you had to write your config file from scratch and some API functions were missing). I refactored my code to PDO (mysqli was missing back then). It worked fine from the beginning, and the uptime and overall performance is really great.
HipHop took PHP code and basically turned it into a statically compiled C++ application. The resulting binary acted directly as a web server. It was pretty fast, but very limited.
HHVM is an actual JITing virtual machine for PHP. It's still huge (one of the project pages say that the binaries are nearly a hundred megs), but it's now a real-world reimplementation of PHP, capable of doing anything PHP can, including eventually being run by mere mortals with normal code.
I'm pretty happy with it. Is it worth it? Well, if you've got a test suite, run it through HHVM to see if you'll have any issues. If the answer is no, then why turn down free performance :)