Practically speaking, the pros of fixed (IMHO):
One less brake to work with/worry about, etc.
Cogs are pretty simple
Easy trackstands

Cons:
Potential for stripping the hub if I'm lazy/stupid
Pedal strike
Toe overlap
Bunny hopping is harder
Harder to use single-sided clipless pedals (LOOK, etc.)
Difficult to explain to co-workers (I can slow down by resisting by the pedals... "Like a kid's bike?" No.)