I think both 590 and 6990 are great solutions.
IMHO they're solutions to a problem that doesn't exist. A dual GPU card simply isn't needed anymore. Anyone buying a 700$ graphics card is probably going to be puting it into an 1155/1156/1336 motherboard all of which support crossfire and sli and unless you have no idea what you're buying, have the necessary second PCI-E slot.
One of the hardest part of GPU design is the two slot size limit. This is what causes the thermal nightmare. I really don't know why anyone would buy this instead of a pair of 570s to put into SLI. They would be cheaper, faster, cooler, and quieter. What's not to like? The same can be said for the 6990.