All modifiers have their uses. However, by far the single greatest problem in selecting and using one is that the design of the modifier does not match the photographer's intent, due to a misunderstanding of what that modifier does. In other words, almost any modifier does SOMETHING to the light--the problem is about choosing an appropriate one.


Of course, the above applies without consideration to cost. More than one type of design can achieve the desired result for a given situation or range of situations, but some solutions may be cheaper or have other advantages not related to the imaged result (such as portability).


That said, my personal opinion is that the hard plastic modifiers such as those sold by Gary Fong and Sto-Fen are very widely misused. In fact, of all the times I have witnessed others using one (they're kinda hard to miss!), I feel they were not understood by the user in the vast majority of cases. The most obvious example is the use of these modifiers outdoors. This is almost completely ineffective and a waste of flash power. In order to be effective, they must be used when there are neutral-colored surfaces nearby to reflect the diffused light. So even when one is faced with an indoor flash situation, the diffuser-type modifiers don't necessarily work any better than a bounce-type modifier. Indeed, it can be quite difficult to control some diffuser-type modifiers because they indiscriminately throw light everywhere, subjecting you to the whims of nearby objects that may impart a color cast.


Another disadvantage of the hard plastic modifiers is that they do not fold up for storage. The "whale tail" is big, bulky, and really cumbersome.


The basic rule of lighting is this: light must reach your subject, reflect off it, and enter the lens in order for you to see it. Yes, it's deceptively obvious but it carries several important points:
  1. If light is cast off in a direction where it won't eventually find its way to the subject, the camera won't see it.
  2. If light reaches the subject, it isn't guaranteed to reflect off in such a way that it also enters the lens. Shiny objects are an example of this phenomenon (indeed, one might think of "shiny" as being defined by this characteristic).
  3. The further (non-collimated) light has to travel to reach the subject, the less its contribution to the exposure. Since we're not doing laser holography, this rule applies.



In summary, I can't advise for or against a specific modifier, because it all depends on how you intend to use it, and what the desired effect is.