AnandTech tries out the 3 games which use DX10, and test them in both DX10 and DX9.  While the features that DX10 offer sound quite impressive, and don’t look too bad either, you take a hit in performance because of them.  In some cases, it could simply be a matter of driver maturity, because there are some situations where you do see a performance benefit.  At the end of the review, AnandTech makes a good point about the speed needed to be displaying 4 megapixels worth of detail on the screen in a FPS, but don’t just skip to the end of the article.
“When it was drafted, DirectX 10 promised to once again change the way developers approach real-time 3D graphics programming. Not only would graphics hardware be capable of executing short custom programs (called shaders) on vertices and fragments (pixels), but developers would be able to move much more high-level polygon work to the GPU through geometry shaders. Pulling polygon level manipulation off the CPU opens up a whole host of possibilities to the developer.”

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