I use Gary Fong's with my FL600R and so far I've been really impressed with it

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My FLICKR
RX100
OMD|EP5|EP1 + Olympus 9-18, 17, 45, 75, 75-300
M9 + Summilux 35 ASPH FLE + Summilux-M 50 ASPH
Zuiko 24 f2.8, Zuiko 50 f1.4, F. Zuiko 38 f1.8, Contax G Zeiss 90 f2.8
Olympus OM-1, Olympus 35 SP, Rolleiflex 2.8 E, Leica IIIf

These shoot-through diffusers still result in light from the flash going directly to the subject. The more ideal method would be one where you shoot up towards a shiny surface which then reflects it through the diffuser and to the subject.
See this: Flash Diffuser SoftBox For Canon 550EX Nikon SB-800 | eBay
This performs as a softbox giving a directional light, which is very soft and diffuse to light the chosen subject.

I have the Gary Fong Lightsphere collapsible & Sto Fen (which are more omnidirectional) and quite a few other (more unidirectional) modifiers (softbox, umbrellas, flash bender) and each can improve the light compared to direct or simple bounce flash, but each has its own limitation.

Boon

The best diffuser is 'no diffuser'...

http://neilvn.com/tangents/about/black-foamie-thing/

Directional light is more effective than straight-on lighting.