Flash bounce diffuser for speedlite

Those are for indoor not outdoor use.

Softening flash light works by increasing the size of the light source relative to the subject. A bounce adaptor (like the one linked) basically works by spreading some of the light out at all angles from the flash head. The light then bounces off walls around the subject and reflects back onto the subject. Essentially it aims to make the walls the light source relative to the subject and thus increases the size way and above that of the little flash head.

Outdoors it will only take a little of the flashes power as there won't be walls nearby to bounce it off; so the effect would be the same as just lowering the flash power a little. Of cousre if you turn the flash head and aim it at a white wall whilst outside then yes you can use that for some softer light from that direction

If you want softer outdoor light you want to look at things like soft boxes or umbrellas. There's a range of small on-camera softboxes (eg by lumiquest) which give a modest increase in softness whilst being useable on-camera.
Meanwhile if you get off camera flash you can use larger boxes/umbrellas - you might well need an assistant or tripod for those however.

Turing the flash head to aim it at a reflector (just like the wall) can also be done. You can also use a reflector to shine natural light up at the subject instead of using the flash. The bonus being, again, that the reflector is a lot bigger.

 

Using things like this outdoors (or in a large room) is a very common thing to see....but it's also a bit ridiculous because they don't help and will actually make things harder for you.

Lets examine what they do.

A flash is very efficient at putting light out in one direction. If you point the flash forward, it will zoom and match the field of view for your current focal length, giving you just enough light to cover the view. The problem with this might be that light direct from camera to subject will tend to look flat, it can give you red eye etc. It can also give your subjects ugly shadows if there is a wall behind them (especially if you turn the camera to portrait orientation).

So because of those issues, people are more than willing to spend money on these little hats for their flash...but what do they actually do? Accessories like this diffuse the light. They take the directional light from the flash and spread it out. Usually, most of the light will continue in the 'straight out' direction, but the rest will be sent all around.

Now, we should ask ourselves if this (on it's own) improves the quality of the light. Does it make the light softer? Not really. The only way to make light softer is to increase the size relative to the subject (so physically larger and/or physically closer). These hats don't directly make light significantly softer. Do they make the light more directional? No, the light is still coming from the camera position.

So what do they do that can help? The intended use for these things is when you have surfaces around you to bounce the light off of. Walls & ceilings etc. You don't need one in order to bounce the light from a flash, you can do that just by pointing the flash at the bounce surface. This alone can give you soft & directional light. Used well, you can create studio quality lighting simply by bouncing the (bare) flash off of a wall etc. So what do the 'hats' do? As mentioned, they allow most of the light to continue, which you could use to bounce the light, but it also sends some out all around...which, when bouncing, will probably include 'direct to the subject'. So these hats are good at splitting up the light. Most can be used to bounce, and the little bit that goes direct to the subject, can act as fill to the bounced light. If you are in a small space with walls all around the subject, you might get some light bouncing off all of those surfaces and back to your subject (and/or their background etc).

So, the primary use for these things should be when you are shooting with bounce surfaces around you, and you want to bounce the flash off of those surfaces, but you also want to send some light directly to the subject, and/or if you want to bounce light all around.

So if you are shooting outdoors, with (presumably) no handy surfaces to bounce off of....would you use one of these hats? A lot of people do....but really, they shouldn't be. On it's own (without bouncing) they really do very little for you. And if you point the flash up (or at 45 degrees) (I see this all the time) then where is the light going? Most of it ends up going up to the sky (wasted) and the only flash light they get in their photo, is that tiny portion that gets spread out and sent directly to the subject.

In other words, they are likely being very inefficient with their flash, while not improving the quality of light. Being inefficient means that the flash has to work harder (the TTL auto metering will make the flash fire with more power). This means that each flash burst has to be much stronger than if the flash was bare and pointed straight ahead. This in-turn means that it will take longer for the flash to recycle between shots. This likely means that you'll miss some shots because the flash hasn't charged up. This also means that your batteries will be drained sooner, so you'll have to swap to new batteries more often, you'll have to carry more batteries etc.
(plus, to anyone who understands light, they look like idiots).

So feel free to spend your money on a flash hat, or one of those noodle cup things. But understand where to use them, and where not to.