Its not only loud, its huge, and takes up 1 PCI slot. Its just not an elegant solution. One might argue that you probably can't fill up all your available PCI slots, but what if someone can? nvidia shouldn't take away that that option from any user.
Then there's gonna a debate that nobody's forcing anyone to buy a GeForce FX, and that's true. I sure as hell am not gonna get one either, but IMHO its just not a cool(pun intended) solution. It might work for an ultra high end model, but nvidia's apparently stick with Flow FX on both the GF FX 5800 Ultra AND the regular GF FX 5800. What's up next? Water cooling? Flow FX2 that takes up 2 PCI slots?
Anyway, I think it'd be better to use a 1U cooler on the GeForce FX than that Flow FX. It looks better, sounds less noisy, is lighter and probably performs as well if not better too.