Seriously why would people buy 6990 when its so freakin expensive
Let's compare to two 6950/6970's:
-Takes only 1 PCI-e slot
-You can fit up to 4 of them for a total of 8 GPU's on a big motherboard
-You don't need two computers to run a big amount of GPU's
-You save by not having to buy another (or multiple) motherboard, power supply, processor, cooling, RAM
-Consumes 375W at stock 450W in OC mode; two 50/70's consume 500W in total without any overclocking (much more if OC'ed)
-Better resale value
So it's actually a bargain and one of the best, if not the best mining card for large clusters.
Of course it's not the best card if you only intend to run one PC and earn a few bucks per day. I however bought 10+ because of the above qualities.