We have one of that EPU-4 capable ASUS motherboards in the "playroom", coupled with an MSI-8800GTS. According to the Asus EPU-4 utility, it is achieving power savings at time when we are not fragging our lives away over CoD 4. :)

Without a power measurer, it's hard for me to determine if there are actual power savings, but the EPU-4 utility does function similarly to an automatic over/down-clocking software.
Also, it's just ASUS marketing FUD that says only ASUS branded graphics card can benefit from EPU-4 - any graphic card should be able to benefit from it. ASUS is just saying that they tested their OWN cards to confirm support with EPU-4.