Are small on-flash softboxes useful, or a gimmick?

Question

There are a number of "mini-softbox" products on the market. These are meant to mount on a hotshoe flash (usually with an elastic band).

_{Lumiquest Softbox III}

Are these worthwhile portable and low-space light modifiers, or are they more gimmick than useful? Are they large enough to meaningfully diffuse light for portraits? Are they too close to the flash?

Are any particular designs better than others?

Answer

I have the Lumiquest Softbox III that's mentioned, and I find it useful as a super-portable softbox that's better than nothing. Given the option to have a huge softbox that would be my first choice, but the small softbox, placed very close to a subject, works really well and provides a much softer directional light than one would get with a bare flash or with just a simple diffuser such as an Omni-Bounce.

Check more discussion of this question.