On-Flash Diffusers: Useful or Useless?

i have this one http://www.lumiquest.c​om/products/promax-system.htm and for me is the best...

It depends on what you are doing and what kind of diffuser.

Soft boxes and brollys work well as diffuse light sources.

Things like the stofen and fong are useless when used outside (basically they rob you of flash power, making your flash work harder, get hotter and chew through batteries quicker
but they can work well when used inside.

Twice this week I have used a stofen on my flash, in a multiple flash setup.
Flash/stofen/pocket wizard stuffed on top of a locker or doorframe to light the room as a fill light.
Then another bare flash as a key light to just illuminate the subject.
Unfortunately I can't post examples Due to a copyright loophole with work (I can't reproduce it without permission)