Wow those look really tempting now. Pretty close to a 5970 / 6990 but without the power, heat, or noise. But as was said, they're only *just* slightly more efficient for MH/$.
It's all about reducing the power bill for me. Same cost (there abouts) for the same hashing power, but at greatly reduced power consumption. I pay a lot for electricity though so it makes sense for me. If you pay peanuts for your power then I can see how you'd not be quite as tempted.