Here is a real world example of the difference a great diffuser can make: A very inviting water cooler, just waiting to be photographed, complete with condensation. The camera and flash were rotated to portrait position with flash to the left.
The first picture was taken with direct flash. The shadows and bright spots are less than desirable. Typical of direct flash.
The second picture was taken with a Graslon 4100F Flat flash diffuser. Notice the smooth and soft shadows, and the more natural looking highlights.
The third picture was taken with the Graslon 4100D Dome flash diffuser. Even softer.
This is a great example of the perfect use of the Graslon 4100. Close shot, not much natural bounce (ceilings were high).
Softer light. Better pictures.