Making the light 'softer / less harsh' is accomplished by making the source of the light bigger, in comparisons to the size/position of the subject. So with that in mind, most of the flash accesories don't really soften the light on their own. I find it kind of funny actually...because I see a fair number of people who put a Stofen Omnibounce onto their flashes and fire the flash directly at the subjects. The Omnibounce is practically the same size as the flash head, so it's not increasing the light source at all....it's basically just blocking some of the light, making the flash work harder.
The Omnibounce, the 'Fong Dong' and most other flash accesories...are made to help when you are already bouncing the light. By bouncing the light, off of the ceiling for example, you are making the ceiling the light source, which is much bigger than the flash head...so you get softer light. The problem with that, is that all or most of the light is coming down onto the subject, which can make for bad shadows. What the accesories do, is separate the light...letting most of it pass through for bounce...but also throwing some of it forward to fill in the shadows. The Fong sphere throws light in all directions...and if there is something to bounce off of, then the light comes back very soft...but if there is nothing to bounce off of...the light is wasted.
A simple bounce card can do the same job as most of these. You point the flash at the ceiling (or wall etc) and bounce the light. The card directs some of the light forward to fill the shadows.
Of course, there are some accesories that actually do increase the size of the light source, which softens the light. But of course, the bigger they are, the less convenient they are to use.
As with anything, you can't rely on any particular accessory to automatically make your shots better. The best accessory is a solid understanding of lighting...then you can figure out when and how to best use your flash and any accessory that you might want.