Performance Improvements:
As with most Catalyst releases, performance has increased in various situations.

Far Cry performance improves 4-5%. Larger gains of up to 18.3% can be seen when using anisotropic filtering.

Fear performance improves as much as 8.5% on X1800 and X1900 Single card systems and as much as 14% on X1800 and X1900 Crossfire systems.

Splinter Cell (original) performance improves as much a 7% on X1300 and X1600 Crossfire systems.

Issues Resolved in Catalyst Software Suite 6.6:

City of Heroes: Display corruption is no longer noticed when playing the game under Windows XP with an ATI Radeon X1x00 series product.

Quake4: Texture corruption is no longer noticed when playing the game.

Age of Empires III: All the buildings and silver mines no longer shimmer at the same time when right-clicking on an object.