In order to baneling rush effectively, you have to make sure you are putting yourself in the appropriate position for it and that your opponent is not going to counter your attack. Certain units are very well designed to deal with banelings whereas others are going to fall much more easily. In order to understand exactly what you need to do, here are 3 of the best situations in which to use a baneling rush against a Starcraft 2 opponent.
Against Terran infantry, there is hardly anything in the world more effective than a group of banelings. These osf my chart units will swiftly destroy even a large group of infantry units. The reason is because they deal a ton of splash damage, which is extremely effective against slow run of the mill Terran infantry tactics such as MMM. Definitely use a baneling rush in this situation!
Zealots are one of the most common victims of the baneling. Their nature as a melee unit without a sufficient amount of speed makes them a perfect target for a baneling attack. If you see an opponent massing an army that is primarily focused on these units, you are going to want to get in with a baneling rush as soon as possible to take them out!
Standard Zerg infantry falls almost as quickly to a baneling rush as does the MMM build with Terrans. The reason is because of the splash damage. Zerglings are particularly vulnerable to it due to their low health. In addition, Hydralisks are not particularly fast or resilient to a baneling rush, so using it against them is also an excellent idea.