I try to never use nukes, but I did find them useful a few games ago. I was second in population, but first in technology, and was building a spaceship. I saw that I would win via the space race, but did not want to finish behind another civ in pop, so I built nuclear subs, tactical nukes, and ICBM's. One turn before the space ship was finished, I annhiliated my closest competitor--nuked every city (around 25). I remember that the Mongols also declared war against me, so I threw them a few glowing bones for good measure. When I won, I had rocketed (pun intended) ahead in population, and won the game on all counts.
Generally, though, I only have nukes as a deterrent. The results (in both reputation and pollution) are too serious to take lightly.