I can only see two reasons that anyone would buy a Radeon HD 6990, given the other options currently available.
First, because this is a dual-slot card, you’re able to fit two of them in an ATX chassis and enjoy the potential of quad-CrossFire. The list of problems with this prospect is significant, though. You’re dumping half of the heat from each card back into your chassis. That’s unsustainable to the point where boutique system builders are simply failing the config, choosing not to offer it at all. Enthusiasts with a lot of money are sure to scratch their heads, wondering why such a setup isn't available. The short answer is that it doesn't result in a good experience. And if a single card is capable of making the GTX 480 sound like a kitty cat's purr, a pair would have to be unbearable times two. I’d quantify my guess for you, but AMD smartly only shipped us a single board. Instead, I have to rely on the expert testimony of builders who’ve already tried quad-CrossFire and are reporting noise levels about six decibels higher than with one card installed.
Second, you really want access to five display outputs. In that case, please don't let me dissuade you. The Radeon HD 6990 certainly has a rich I/O suite that Nvidia can't come anywhere close to matching today. And the single-GPU Radeon HD 6900-series cards "only" give you the option to connect four screens simultaneously, for now.
Chase Is A Race, Nvidia
AMD does get to claim the title of fastest graphics card in the world—an honor it already held with the Radeon HD 5970. But this was the company’s big move. Nvidia is guaranteed to give chase sooner than later. And then we’ll have a race.
Based on what we know about the speed of GF110 versus Cayman, in a vacuum, Nvidia has a distinct performance advantage. But its thermal boundaries with that chip are even more pronounced than AMD’s. So, milking more speed out of a dual-GPU card without running head-first into the same cooling/noise wall will be a real challenge.
Should Nvidia choose to cool its card like the Radeon HD 6990, it’ll make chassis choice and cooling fan orientation a real pain in the butt for boutique builders and do-it-yourselfers alike. In that case, we’re probably looking at another situation where four GPUs in the same machine will be problematic. Hopefully, Nvidia sees the situation AMD is dealing with today and reevaluates the decision to let professional builders and enthusiasts figure out how to cope with the heat pushed out the front and back of its products. I'm not going to take "buy a Cooler Master HAF 932" as the only acceptable solution for integrating hardware that could have been engineered to cope with thermal output more intelligently.
We Have A Better Solution
Given almost-embarrassing fan steppings, multiple attempts at functioning CrossFire profiles, driver anomalies, and a general reluctance to talk about the logistics of actually selling the Radeon HD 6990, I get the sense that AMD rushed this launch. Marketing 101: it’s better to claim first-place today and possibly get passed up tomorrow than have to launch a second-place card tomorrow.
So, we end up with a flagship board that is undeniably fast. It’d really be an all-out rock star if performance were our only consideration. As it sits, though, heat and noise are this card’s enemies in the same way they plagued Nvidia’s GeForce GTX 480.
Those detractors might even be forgivable if there weren’t better options available. Let’s just assume two of these in quad-CrossFire isn’t even an option for most people, and you’re looking at the best possible way to leverage two GPUs. A pair of Radeon HD 6970s sacrifices nothing except space in your case. That combo is faster, quieter, and more effective at dealing with heat. A pair of GeForce GTX 570s would be a great alternative, too. You give up a bit of speed in some situations, but they’re even quieter than the 6970s.
And as I write this, one day ahead of the launch, AMD refuses to comment on pricing. There’s also no official word on when enthusiasts will see the 6990 available to buy. We’re not worried, though. Skip the Radeon HD 6990 altogether and take advantage of fantastic CrossFire and SLI scaling using more elegant single-GPU cards.
Update: Hours before launching the Radeon HD 6990, we're happy to report that AMD decided on a price: $699. That's $20 higher than a pair of Radeon HD 6970s, not counting $40 worth of rebates. AMD counts the three adapters bundled with the 6990 as a $60 value, but we maintain that the two single-GPU boards in CrossFire are a superior choice.