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There was quite a lot of buzz about it during Haswell release. Since I was an early adopter I had no time to check it properly. Now I can confirm that my PSU - Corsair CX430M - is definetly not compatible. I kept C6 and C7 enabled for over a month. During that time I experienced random segfaults in various apps, from intel video driver and OpenGL apps (most often), through compiler errors up to kernel panic (most rare - only 2 times - but each time with partition corruption). I also noticed that most errors happened right after cold boot (yes, I turn off my machine when I'm not using it) and most times they vanished after two or three reboots. After disabling C6/C7 everything runs smooth and stable. I know that these states are designed for laptop battery run times but I'm curious if there will be any difference in electricity bill with them enabled/disabled on a desktop machine.

You probably can save 2 watts with C7 mode when idle. For saving one kWh, you need to idle 500 hours (21 days) 24/7. Assuming electricity cost 0,25 euros per kWh, that makes 17 kWh or 4,25 euros per year. As Poland currency 17.8283 zlotys power year. However that calculation requires you to idle 24/7/365.

Buying good enough PSU would cost like 70 euros, so you must use it 16 years to break even. Not worth it. Consider that C7 state to be just marketing BS on desktops, nothing more.