The "fish eye" lens is so often overused, or not used well, it's worth mentioning.
A fish eye lens is for...
Emphasizing the weird, the bizarre, the exaggerated.
This means both the emotional quality of the event (i.e.: a crazy street fight or dive bar dancing) or the scope of the event (i.e.: a giant crowd protesting, one person in a cramped space, etc).
Its function is to turn real life into a cartoon: it's best to focus that emphasis toward a meaning that adds something to the image.