I think guided missiles are are quite useful in the right situations. As part of a naval stack they are essential: firing a salvo of missiles at an opposing naval stack will often decide the battle if you have brought enough with you. But the best use I've ever gotten out of them was actually on land during a multi player game. I used them to kick off an attack on a neighbour that was getting close to a culture victory win. Because of the map layout I could only reach one of his three culture cities, and he put every mech inf he had into defending it, along with every jet for air cover. He had seen me coming before even I did! So at first I thought it would be a blood bath attacking the city, until I realized that I could put as many missiles as I wanted into my city opposite my target and soften his defenders with one large bombardment of missiles instead of using air power. I had a large navy that was useless to me for this war, so I stripped all the subs and missile cruisers of their missiles and had every city building them for a few turns before the battle, and re-based 40 or 50 missiles into my staging area the turn before attacking. It worked better then I would have thought: all of his best defencive troops had taken damage, and I was able to take the city with relatively light casualties.