When doing alot of running and gunning with your flash in the hotshoe, it is a much better alternative than bare flash. I prefer it over the Gary Dung dome thing that everyone has that just scatters the light around. The window up top allows for a certain control of bounce light vs fill light instead of just dropping a grenade of light a-la Gary Dung.
If you are indoors and setting for a quick portrait, always look to see if you can take it off and bounce the light, which can give more interesting shadows.
Oh, and look into getting their velcro strap to fasten it on. I think the thing ships with velcro tape you actually stick on your flash. Avoid stickign things on your flash as much as you can, as this will cause problems when trying to put on other accessories.