From what I've read, Nvidia is banking on developer support to make this card a winner. On the games where Nvidia has thrown some money into the development of the title, the GTX 590 shines quite well. On the other games, the HD 6990 is ahead, sometimes by as much as 30%. Guaranteed this is due to the extra memory on the 6990. The GTX 590 seems to run out of steam on higher resolutions.
For right now, the HD 6990 is faster while the GTX 590 is quieter. I have no idea where we will go from here, but as predicted, the market is now divided between most powerful and most "acoustically friendly".
I'm anxious to see how AMD answers this. Perhaps they will redesign their cooler to be more inline with Nvidia's design.