Has anyone used HipHop for PHP?
Yes, my team was there at Facebook HQ before release and have messed around with it here and there since. There are a number of things that simply don't work in HipHop.. conditional includes, eval(), and other similar aspects. Remember, that this is more of a way to improve performance via a better deployment method.
If you only have a single server, definitely don't use it. Deploying the compiled app - which can easily hit 1GB - takes your server down for a brief period. If you're using a handful of servers, it probably doesn't make much sense either... a 50% cpu load decrease doesn't save you much.
If you have dozens or even more servers, then it makes lots of sense.