I don't know if that particular modifier is worth the money, but it is a completely different thing from the diffuder that came with your flash.
The included diffuser is intended to ensure that the flash covers the field of view of very wide-angle lenses (something the flash reflector zoom alone cannot do). It is not intended to soften the light in any way, just to spread it out more. The shadows and contrast will still be just as harsh (except that there may be a little extra fill from room reflections).
Some Nikon flashes (notably the SB800/900/910) come with a milky-white dome diffuser that is intended to provide something like a bare bulb effect, sending light off in a nearly 180-degree hemisphere to take advantage of room reflections.
Softboxes (such as the device you linked to) and bounce cards (and similar accessories like the Lumiquest Bounce) are intended to make the light source bigger in relation to the subject, softening shadows and reducing the contrast between dark and light areas. Umbrellas are used for much the same thing. If you have to use an on-camera flash, then a softbox can vastly improve your flash pictures. Keep in mind, though, that they're just as useful off-camera, and getting the flash off of the camera when you can (or bouncing the flash off of a nearby wall or ceiling) can do much more for your pictures than an on-camera softbox.