FUD Zilla claims CrossFire X and Quad SLI doesn't really works as it's supposed to in most DirectX 10 games:
We've learned that Quad Crossfire and Quad SLI will have some performance challenges with many DirectX 10 titles. We expect that CrossfireX might have some scaling issues with titles such as Crysis and a few other Nvidia loving games. It will just be tough to get some extra juice from three or four cards.
We learned that under DirectX 9 the games can put three or more graphics card to good use much easier, while in DirectX 10 the same titles won't scale that much.
CrossfireX driver is scheduled for the first half of March but we are sure that in the month to come there will still be much space for performance improvements.