Do you know what situations you will be using your flash in?
I ask because most flash accessories work well in some situations but not so well (or not at all) in other situations.
To make light softer, you can only do two things....increase the size of the light source, or move the light closer to the subject. While some flash accessories make the source bigger, it's only a little bit bigger, which doesn't really help all that much. And the ones that do significantly increase the size, are well, big and hard to wield.
The easiest way to soften the light from your flash, is to bounce it off of a ceiling or wall. And that's actually where most accessories help most. A typical problem when bouncing, is that all the light then comes from above, giving you shadows like you might see under mid-day sun. So the accessory splits the light, sending some forward while letting the rest bounce. Some of them, like the LightSphere, spread the light out all around, in the hopes that there is something to bounce the light off of. It works well in smaller rooms, but is utterly useless outdoors etc.
There is a time & place where flash accessories can come in handy, but knowing how to use your flash is much more important. If you do use an accessory, it's important to know when to use it, and when not to.