Oh no, here we go again.
Anyway, here's a test of a bare flash and a Sto-Fen. The camera is on a tripod with a remote release. The flash is pointed directly at the test subject.
First, the bare flash:
Now the same shot with the Sto-Fen attached, all other settings are the same:
As you can see, the light from the Sto-Fen is clearly more defused. Look at the shadow under the chin of the test subject.
Is it ideal? Probably not. Does it work? Yes. Is it worth the $11? I think so.
Now, when you actually use it the way it's intended to be used (bounced) I don't like the results as much as just using a bare flash (bounced). Example:
Bare flash bounced:
Same setup and settings, just with Sto-Fen:
When used "properly" it doesn't improve the light IMHO, it makes the shadows more harsh.
Here's a real-world shot taken with a Sto-Fen attached to my flash with the flash pointed directly at the subject:
In essence, I find the Sto-Fen more useful when it's being "misused" than I do when using it properly.