Slow, expensive, lightly armored, and far too specialized, Missiles are rarely seen units due to their lack of any viability.
Missiles are designed to destroy Air units, either instantly destroying or dealing very high damage to any Air units that venture into its range. In reality, this rarely happens, as all Air units have higher movement ranges than Missiles have firing ranges. Combined with its low movement speed and bad movement type, Missiles are extremely easy to outmaneuver with Air units. The only practical use a Missile unit has is it can effectively prevent an Airport from being able to build units by moving it into range of the Airport. Generally speaking, however, if your opponent allows this to happen, you have likely already won the battle as Airports tend to be relatively close to other production facilities.
If your opponent actually builds a Missile, take advantage of the fact that they will have much less money to spend on more combat-viable units. The only way for a Missile to get a kill is if you let it, so be sure to check its range before moving Air Units. Missiles take high damage from all units, including Infantry and Air units, so an exposed Missile is easily destroyed. Missiles are easily avoided however, so it may be safe to ignore Missiles unless you need to open the area for Air units to travel.