I'm running a Mac mini (late 2012) with OS X 10.8.2 as a media center PC and streaming server. It's not running 24/7 - instead I'm using a schedule to wake it up in the evening and putting it back to sleep at midnight. So far, so good.
For some reason the built-in VNC server stops working when the Mac wakes up while the attached screen is turned off. I can login and can also move the cursor but the screen remains black.
A workaround is to use RealVNC but I prefer to use the built-in VNC service since it doesn't cost extra money and has better multi-user support.