When I tried it I was amazed by the performance impact.
Just create yourself a PHP file that does 100.000 calculations and
compare the php execution vs. the native compiled one. That is awesome!
The drawback is, that hiphop does not support every php extension and
that it really takes some time to compile a really big framework.
If you have some code that you want to speed up and it can seperated
from the rest, then create a smaller module out of it and compile that.
Or think about creating own PECL extensions.