I tried it two days ago after they made the fastcgi announcement. My day job sites are fairly big with what I consider a pretty large number of contrib modules. I managed to get it up and running, but the HHVM logs were full of notices and it finally bombed out after several requests (not enough to really warm up the VM) on something related to apachesolr, and another one related to GD, which is strange because we use imagemagick, but whatever. Sad Trombone.

The really cool part though, it truly was "drop in". Every bit of my existing nginx/fpm configs worked as is, so once it's a bit further along, it's just a matter of shutting down fpm and firing up HHVM.

There were also a couple of gotchas in the setup related to the fact that their apt installer installed HHVM configs that don't assume Fastcgi, so it took a little bit to figure out the correct edits to the init script and HHVM config so it'd listen on :9000 instead of :80, but all in all that it worked at all was really fun for a minute. Very excited about this project.