I can't talk from personal experience but the reviews I have read in regards to the card it that its dead quiet. A very efficient cooling solution but thats to be expected from the bloody size of that heatsink.
I've never owned one, but I've seriously considered buying it. The YouTube performance reviews I've watched state that they are quiet though. The second link I posted has a sound demonstration. The other reviews are other info about the card. I should mention that this card is huge and probably heavy. You should very that your case can handle the size, and you may want to invest in a GPU bracket for it.