I decided to capture a post that I made to Nikonians.com for the Flash Forum:
The question I was answering was that the person had tried verious diffusers and was asking why he didn't see much difference between them. The bottom line is that there isn't much difference between them until you put them in specific situations.
First, you have to understand exactly why diffusion is needed in the first place. The basic problem is that all the light from an undiffused flash comes effectively from a 'point' source. This means all the light hits the subject from one direction, and that's what makes it harsh. To soften the light hitting your subject, you have to make it hit your subject from multiple directions. This causes tiny shadows to form in multiple directions, that merge into one another adding definition to facial features giving your subject a more natural look.
To make the light from the flash hit your subject from multiple directions, you have to use a diffuser, and the bigger the diffuser, the more paths the light will follow while traveling to your subject, and the softer it will be.
When you are indoors and have a white ceiling (that is not too high), and white walls, you can make your flash really soft if you send the light in all directions, so it bounces off every wall and ceiling before arriving at the subject. Then, you get amazingly soft light.
So, any diffuser that will send the light in all directions indoors in a room with white walls and ceiling, will make equally soft light. The small snap-on diffuser that comes with the SB-800 will do a remarkable job of this if you simply point it straight up.
However, there is more to this. In addition to making your light soft, you normally want to send a bit of light directly toward the subject to make catchlights in the eyes and lift shadows on the face. This is called 'direct' light, and if the direct light is coming from a tiny source, then you are adding harsh light into the image, which cancels out some of the softening you have created with the bounce.
So, the overall quality of the light on your subject depends on controlling the softness of both the direct light and the bounced light and the ratio between them.
So, let's look at the various diffusers you mentioned. If a bounce card is used without any other diffusion, you may already see that the direct light will be coming from a source that is the size of the bounce card. If this card is small, then the direct light will be harsh. This is what happens with the pull-out bounce card in the SB-800. It makes very harsh direct light, so it provides no diffusion at all by itself. Only the light that gets around it and is allowed to bounce off walls and ceiling will be soft. You can make it do a good job by pointing it up and adjusting how much of the bounce card is sticking out.
With the Demb flash diffuser pro, it uses a larger bounce card which softens the light more than the small card in the SB-800. You can compare softness of bounce cards by comparing the area of the bounce cards. The SB-800 card is 1.4 x 1.75 or about 2.5 sq inches. I don't own a Demb Flash Diffuser Pro (DFDP), but from the pictures it looks like its bounce card is about 3 x 3 inches or 9 sq inches. This will provide slightly more softening of the direct light than the smaller SB-800 card. Then, the DFDP adds a 'Front Diffusion panel'. This adds about 2 sq inches to the overall size of the direct light source and will cause part of the light to scatter, assuming an inverted funnel shape towards the subject. By itself, this diffusion panel will not add any softening, because the active area is about the same size as a flash. However, if you are indoors, some of the scattered light may hit the walls and arrive back at the subject. This light will be soft, but much of it will hit the subject from the sides, so it won't soften the face that much. The DFDP is a very versatile system that can produce great results once you fully understand how to use it.
The Lumiquest Mini Softbox softens only the direct light. It produces no bounce, so the area of the active surface of the box can be used to compare it to bounce cards. From the website, it says it will fold flat to 3.25 x 4.5 inches. This is about the same size as the active surface of the box so the area is about 15 sq inches. Therefore this will provide more softening of the direct light than the DFDP.
Then there is the Light Sphere II (LS) which comes with its 'inverted dome'. With the dome installed, and with it pointed straight up, it will cause the flash to scatter in all directions. Light will then bounce off the ceiling and every wall in the room, just exactly like the small snap-on diffuser that comes with the SB-800. This makes the effective light source the size of the entire room. This could be a total effective area of 500 sq feet. Compare that size to any bounce card there is and you see that there is no comparison. You get extremely extremely soft light, probably only to be outdone by an overcast day, when the entire sky becomes the diffuser size.
However, the advantage of the LS over the small snap-on diffuser, is that its active cross-sectional size is about 3 x 4 inches or 12 sq inches. This means that its direct light will be much softer. The direct light from the LS is not as soft as the Lumiquest Mini Soft Box and about the same as the DFDP.
The real power of the LS is that when you have a good ceiling bounce, you can remove the inverted dome and take full advantage of it. This is when you get maximum softening. Not only do you get a maximum bounce off the ceiling, but you still get a bounce off the walls and nice soft direct light as well. You also have the option of tilting the LS backwards to reduce the direct light while still retaining maximum room bounce. This is why the LS works so well.
Lastly, I should point out that as you move farther from your subject, the apparent size of the source grows smaller and smaller. This makes the direct light get harsher and harsher. Once you are beyond about 15 feet, none of the diffusers will have much of a softening effect on the direct light. When you are 15 feet or more away, you have to rely entirely on the room bounce for softening. Then, if you are outdoors (and within 15 feet), where there is no bounce, you have to rely entirely on the softness of the direct light. This is when the Lumiquest Mini Softbox beats the DFDP and the LS.