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Having read all the good comments I bought the Demb but was very disappointed by the results. The area of light is still small, so you still get an effect very like direct flash.
I decided it was no better than the Nikon diffusion dome that comes with the SB-800.
I got better results with a cheap white bag thing from China (from eBay). You poiint the flash straight up and it goes on the end. It cuts out a bit too much light but gives a good result for closeups.
Simply pull out the white card built into the SB-800. Works great. It is my preference.
What a bounce card or plastic diffusion dome does is to provide some direct flat frontal spill for fill to assist the bounce. Also adds catchlights in the eyes, which is good thing. The size of this card determines how much fill. If you use a large card, all you get is lots of flat direct light, overwhelming the ceiling bounce. Might as well just use direct flash then, and save some wasted bounce power. People greatly overdo this, but "more is better" is not often true.
In greater distance cases where you need more reach, Walmart craft department (and other craft stores too) has 33 cent 9x12 inch sheets of thin white rubber foam, which is easily cut down and attached with rubber band. Works great. But not too large, leave something for the bounce to do.
Study your pictures, and think about it, and examine the degree of direct flat frontal fill added. A little goes a long way, and the SB-800 pull out card is perfect for many situations (at less than maximum subject distance). Sometimes pulled out half way is the answer. If you cannot see any shadows from the bounce itself, you used way too much card size.