Please see the most recent updates in the "Where did the .com name go?" thread. Posts number 16 and 17.
DX10 does not allow AA to be enabled through your graphics card control panel. The native AA in FSX looks nowhere near as good as custom AA, especially at 1920x1200.
DX10 gives badly flickering textures on runways & taxiways etc. On my system it gives no performance boost whatsoever. IIRC DX10 may also break vsync in FSX.
It is only a preview mode with some very minor graphics improvements but a lot of graphics problems are caused to get those improvements. Also the very jaggy VC shadowing looks absolutely horrible IMHO.
DX9 for me since it has none of the issues and performs exactly the same on my system.
