I've never used one until now, since I know all the quests by heart, that is until I reach the outlands. Friend got me into one called "QuestHelper" along with another I can't remember the name of, but it places a red arrow on your screen and points in the direction you need to go, how far away it is and how long it will take to get there. It really helped since I normally still have to lookup the quests while in the outlands.

Also tells you the order to do them to be as fast as possible.