Buffered Shadows
Pros :

They are fast
You can calculate a shadow map once and re-use it for every frame of your animation.
They take opacity into account

Cons :

They can produce self shadowing artefacts

There are quite a few parameters to control buffered shadows :

Buf Size
Buffer Size specifies the size of your shadow map. Shadow maps are a bit like texture maps : a larger size gives more precise results. Sometimes, though, you would rather like a very soft shadow and then it's a good idea to lower the buffer size and add some Softness.

Samples
The default shadow map values will give good result, but if you want more softness in your shadow, it might start to look grainy. To avoid that, raise the Samples value. Don't be shy, it doesn't have a great impact on the rendering time and it will nicely smooth the penumbra areas of your buffered shadows. Common values range from 5 to 32.

Softness
Use this to soften the edges of your shadows.But you must be aware of one fact : adding blur will sensibly offset you shadows from their normal position.

Min Bias, Max Bias
Most of the time you will leave the Bias settings to their default values. You will change them only if your surfaces exibit self-shadowing artifacts. It looks like some dark noise over your objects. In that case, you can progressively raise the Bias values until the dark stippling disappears. You will notice that your shadows will be slightly offset from their original position as you raise the bias. PiXELS3D uses a shadow map algorithm that prevent most self-shadowing artefacts from appearing but it might still happen in large scenes.

Render Once
This checkbox allows you to render the shadow map of that spot only at the first frame of the animation and then re-use it for all subsequent frames. It's a great way to speed up rendering, but it means that no object should move inside the spotlight's field of view.

A note on opacity
Opacity is taken into account by Buffered Shadows, but in a binary way : it's either opaque or transparent. It means that you won't have soft gradations in the shadow maps when the opacity of an object evolves slowly across it's surface. What is it good at then ? For example, it allows you to create windows into a wall with texture maps and have the light from the outside cast shadows inside the house.