The AMD Radeon HD 6990 is a hell of a card. From a spec point of view, it slots right in between a pair of HD 6970's and HD 6950's. However, in many instances I find performance to be right in line with the HD 5970 CFX configuration, or a bit higher. This could be attributed to two reasons:
The card is a bit cheaper than a pair of 6970's, and comes with a handful of adapters. If the 6970's drop in price and/or you don't need the adapaters, then the value proposition can change. I don't have formal noise and temperature testing. However, in my subjective testing (comfort level in my office during testing and gaming), it runs quieter and cooler than the pair of 6970's cards, and pulls less power (according to the specs). If you're in the market for a high-end CFX setup, then the Radeon HD 6990 is an excellent choice.
If already have a Radeon HD 5970 Dual-GPU and were "watching and waiting" to upgrade - the Radeon HD 6990 is a significant performance increase, and is a worthy successor. If you have been running a 5x1-Portrait setup on one or two Eyefinity6 cards, you need to upgrade as well. The performance improvements and updated feature set are big step forward.
If you are running a single widescreen setup, any of the high-end GPUs is overkill. Indeed, the Radeon HD 6990 would be the overkill poster-child. The Radeon HD 6990 does provide Eyefinity performance on part with the HD 6970 or HD 6950 in CFX. However, if you truly want to make use of that horsepower, you need to ensure your CPU is up to the task.
The Radeon HD 6990 is a great addition to the Radeon 6900 series. It builds upon the price/performance ratio pedigree established by the 5800, 6800 and 6900 series - and that is a good thing. I'm glad that AMD has finally released their Dual-GPU part, and I'm glad that they have released their 5x1-Portrait part. I'm ecstatic that they are the same board.
The board does support QuadFire, and according to their press deck AMD appears to have mature drivers. Review samples were scarce, so I wasn't able to test dual cards. However, I'm not sure I would even be able to take advantage of them. My Core i7-920 would be significantly underpowered considering the AMD QuadFire testing used an i7-980X.
Additionally slapping two 375W cards on my 1000W Corsair power supply might just knock it out. You're pulling 750W just with the GFX, and that leaves only 250W. Subtract a 130W CPU and you're left with 120W for drives, RAM and motherboard. I'm thinking I'd have to upgrade to a 1200W power supply to go QuadFire.