I find Stofen diffusers pretty much a waste of time. They don't solve the basic problem, which is they don't enlarge the size of the light source which is what's really needed to kill hard flash shadows.
I have the Fong Lightsphere, and I think it's the best solution you're likely to find for a single flash on camera situation. Having said that, it's expensive, and I see no reason why some of the cheaper copies or home made versions shouldn't work as well. Les McLean posted a home made version a while back showing great results.