Ok maybe it IS just a piece of plastic, but I'd say that this is an indispensable piece of equipment for your flashgun. How many times have we taken a photograph using flash (particularly portraits) and been let down by the harshness of the light? Then worry no more because this little gizmo is the answer to your problem. It softens the light beautifully and you'll wonder why you didn't get one earlier.
A couple of reviewers commented on the tightness, but this item fitted my Canon 430EX II perfectly - stays on without a problem, but easy enough to remove if required.
There ARE more expensive models available, but this is really all you need.
I have one minor gripe and it's not about the product itself. Why are these items not supplied as standard with the flash? Same with lenses, you always have to shell out for a lens hood. You'd think that shelling out £200 for a flash and in excess of £400 for a lens that these items would be supplied as standard. It's like the much-hated phrase 'batteries not included'!
Anyway, get one of these babies right now and you'll not be disappointed, go on - it's less than a couple of pints of beer and more useful!