We use HHVM rather than PHP-FPM on Nginx because it is way faster and creates far less load per visitor. HHVM is incompatible with certain PHP that PHP-FPM (PHP 5.x) doesn't have a problem with, so apps sometimes need edits to make them work.
In our case, we use Civi inside WordPress. WordPress seems to work fine in HHVM, as does our testing with Drupal so far.
I can't find any references to CiviCRM's compatibility with HHVM.
Has anyone tried it? Gotten it working? Should we give up?