I'm running Windows 8, with "wake on USB" enabled in the BIOS, and the USB device is either a Logitech MK320 wireless keyboard, or a Logitech M525 wireless mouse, each using their own USB receiver.
Sometimes, wake on USB works. I'll either manually tell my PC to sleep, or it will sleep on its own after 20 minutes idle, and all I have to do is press a key on the keyboard or bump the mouse a little, and it turns back on. It's so sensitive that my cats have woken up the computer by bumping into the mouse.
Other times, it doesn't work at all. No key presses or mouse buttons do anything, and I have to press the physical power button on the computer case. Both times, the computer seems to have gone to the same sleep state (s3) and has woken up just fine. It can happen either when I sleep manually, or when the idle timer sleeps the computer.
Any idea why it works sometimes, and not other times?