An Epic ground mount is highly recommened for the old world, and places like Dalaran where you cannot fly.

A slow flying mount and the cold flying training are recommended so you can fly anywhere.
When in Burning Crusade or Wrath zones, the slow flyer is faster in the air then even an Epic ground mount.

The Epic Flyer is a luxury now.
It is 280% of your ground speed, but the slow flyer is 150%.
When the slow flyer was only 60%, the difference was huge.
Epic flying is something I'm working on for all toons (14 with Cold + Epic so far), but the Slow Flying + Cold Flying is almost necessary.
Flying trivializes so many quests, speeds up your farming/questing/travel time to a large degree, and allows for all dailies, which is a lot of gold.