Have not tried that one specifically but have used similar.
Sto-Fens, and Fongs, and similar products work differently.
The choice of tool depends on the use you plan. There is more than one way to skin this cat and none of these modifiers are magic bullets.
The one you are looking at increases the size of the light source, has a much larger face area than your flash, while still pointing all the light toward the subject. Small light sources like a normal flash gun create hard edged shadows, larger light sources simply soften the edges. This type of modifier works best pointed directly at the subject. It's a tool suited well for outdoor shooting or fill flash anywhere. I don't know what flash gun you are using but my nikons speedlights focus to match the focal length of the lens when pointed at the subject. They also have a built in lens that pulls out and makes the flash cover a wider area. I would suggest that if you use the Pocket Box that using the flip out lens, or a Sto-Fen inside, will help the Pocket Box do its job.
Sto-fens and the like are generally used to throw light around more than at the subject. The idea is to have the light bounce off walls, ceilings, and other surfaces. As long as you are indoors this is very workable and easy to use and can give you quite a bump in quality, outdoors not so much. What this type of tool lacks is focus. Two problems there: first, the light can become fairly flat and IMO uninteresting, like an overcast day. An overcast day is the ultimate soft box, no shadows; second, flash guns only have a certain amount of oomph, Sto-Fens and the like waste a lot of light which limits their range.
Many flash guns have tilt up heads, many even swivel. Along with the pull out wide angle lens mentioned above there is sometimes a white plastic card that pulls out with the lens too. Using these in combination does much the same job as the Sto-Fen style add ons with more focus. The white card, if used, simply bounces extra light toward the subject. Choosing what you bounce off for each shot is more work and is generally only usefull indoors, but it does several really cool things. Like the other modifiers it makes the source bigger, the wall or ceiling you bounce most of the light off is really big by comparison. Since you are only bouncing off one surface though much less light is lost because the light is at least focused in a limited range. You should also think off this as a bit like a bounce shot in a Pool game, aiming the head allows you to add direction to the light that reaches your subject. that means it will generally look much less like you used flash.
Side note. For all flash photography I use as large an aperture as possible, this gives better backgrounds, softer effect, and better battery life because the flash doesn't have to work as hard.