Planks are ranked #10 in a American Council of Exercise study. It is a great way to build endurance in the abdominals and back, while an excellent muscle stabilizer.
Planks are not the sexiest exercise, but they are one of the most effective abdominal exercise around.
By forcing your body to stabilize itself in one position, planks work abdominal muscles and your back muscles. Planks assist in developing better looking abs because they work the transverse abdominus (that holds in the recturs abdominus), crunches focus only on the rectus abdominus.