I have had great success with the Gary Fong Lightsphere. You can see by the reviews on B&H that is is a very popular light modifier, especially among the wedding photography enthusiasts.
When they work the best for me is when I am in low light situations where bouncing my flash off of a white ceiling is not practical or possible.
They are not going to replace off-camera options, but when you have to stay portable and move around areas, and your camera is already at its highest light gathering capabilities, these are a very valid solution.
I think portrait work is going to be the best opportunity to use these light modifiers. If you are in a close cramped quarters, such as an apartment, you should still be able to bounce your flash or setup a simple lighting kit.