Attaching it: I already owned a couple of speedstraps, so attaching it to my flash (Canon 580EX) was simple. It has four velcro tabs that secure it to the flash. Velcro being velcro it isn't totally secure, but I haven't had it fall off or otherwise disrupt shooting. You could easily secure it better with an elastic on the outside of the tabs, or some other cinching strap.
Use: A large softbox this is not. While it does increase the apparent size of the flash, it is only effective for relatively short distances. For small items, product shots, or a daylight fill flash this is an excellent tool. Above is my first shot submitted using this light as the primary source. I also see using this closeup for kids or people...
Of note is that the softbox III has pretty even light distribution. I find that taken a photo of the front of the box shows pretty even light distribution when the flash is manually set to wide angle. I have used it with both a cord and a ST-E2. In both cases you get the benefit of ETTL exposure, so I haven't bothered to figure out the lighting loss in stops.
While there are a ton of links to make your own similar device, Lumiquest has captured those of us who would rather not spend a bunch of time eating cereal and cutting boxes :)
As you can see in the photo, it has pretty nice light falloff as well. You get few harsh shadows (assuming it is within a few feet), but it doesn't light the whole room like a sto-fen.
Unlike a bunch of other modifiers that promise lots but don't make much difference, I find this lends itself to some specific purposes - and does a good job of them. Mounted to your flash on your camera from 10 feet away you won't notice any difference except a loss of light...