It makes macro photography a breeze. With some simple snaps and twists and presses of buttons you can do stuff that would take multiple lights or remote controlled flashes to do without it. I think if you have a Canon SLR with a macro lens, this is an item you want. If you do some sort of (small) product photography, this is a neat thing to have. I have posted a quick video that kind of shows how it looks and how it can be adjusted.
I prefer to take macro photos with it in manual ("M") mode where I adjust the aperture to get the depth-of-field I want, the shutter speed to both reflect if I am hand-holding (like 1/200s) or if I have tripod mount (slower), and ISO fixed to 100 or 200 for best quality. Then the E-TTL of the flash system simply adjust the flash strength to make the exposure right. It is a breeze. Only drawback is that it is expensive and somewhat bulky.