Bunkers are great for defense, but they are even more effective when used offensively. Find enemy towns and build Bunkers around them. Place Marines in them, and use multiple SCVs to repair them. This works very well against the Zerg, and less so against Terran and Protoss due to Siege Tanks which can kill the Bunkers from a distance, and Zealots which can overpower the Bunkers.

             This sort of strategy should be a worry to players very early in the game, and at expansion towns.

             When attacking the enemy, use this method while the enemy is distracted or when the enemy doesn't have any units to stop it. Rather than sending in 4 Marines, send with them an SCV and Bunker your way into their town. You can start building them out of view of the normal paths into the village, then leap frog Bunkers closer and closer to the enemy town until you're building Bunkers inside it. Combine with Siege Tanks for even more effectiveness.

             The best defense against this is regular patrols outside your towns, making sure the enemy is not setting up around them. Catch the Bunkers before they are completed along with killing any SCVs in the area.

             This preventive watch must also be used to prevent the Protoss enemies from setting up Shield Batteries and Photon Cannons around your town.