It's a good start, but it only makes the area of a pop-up flash a bit bigger than 430/580. As a result you'll get less sharp edges to the flash, but from a distance it'll still look like a spot.
With a large softbox, it loses even more light (which is why I bought the 580EX II) but being a foot across as you can see it filled in the shadows on David Coulthard's face and lit the kitten quite evenly.
I also bought a flash extender for the Canon that lets me get the flash off the camera and still use the ETTL automatic control.

But you don't need a Canon flash if you're willing to adjust things manually.
I admit that could be a problem if you don't have time to setup and plan the shot, but when you're going to follow people in their work you should have time to setup and adjust.

I strongly recommend you visit strobist to see what you can do with a few relatively cheap flashes.