In general, sleep works brilliantly on my hackintosh'd mini 9, save for one issue. If I have bluetooth enabled and legacy usb disabled in bios, when I open the lid, the computer will think I'm pressing the zero key over and over until I hit another key. If I have bluetooth disabled and legacy usb enabled, the computer will think I'm pressing tab over and over. Additionally, with the first combo, if I hit the power button and select restart after waking the computer from sleep, it won't actually restart, but with the second combo, it will. Odd, but certainly not deal breakers.
I have occasionally had the computer fail to sleep, usually when a program was frozen. Like others have said, if the power led isn't pulsing, it's not actually asleep. I don't think this is unique to our minis though, since all of my computers, windows or mac, have failed to sleep at times.