Going a bit off topic here because I totally agree that beauty dishes need to be big to produce the real beauty dish effect, and the question is about tiny ones that fit onto hotshoe flashes.
What beauty dishes actually do is to create shadows - or modelling if you prefer the term - that define and shape the face. Therefore, they are an excellent portrait tool for the right shape of face, with good skin and high cheekbones. Good skin (or extensive retouching) is essential because the shadows show every fault.
I find them easier to use than softboxes and I use them a lot for still life subjects too - they are incredibly versatile.
Read
these 2 articles, one by the famous Marc Gouguenheim and one by the infamous Garry Edwards
I find that they work best when on axis with the model, obviously if the model is looking straight at camera then that's the same thing as being on axis with the camera, but if the model is at an angle then it isn't. In other words, the model should be pointing her face directly at the beauty dish. Again, see the articles, there is an example of the model looking straight at camera and another with her looking straight at the beauty dish in
my article