Declines are also popular. They tend to stress the lower or sternal pec. The biggest execution flaw is that lifters may start out hitting the nipple area, but as the weights increase, the bar tends to hit closer and closer to the waist area, with many lifters doing a 'belly bounce'. This shortens the range of motion considerably. Perhaps this explains why some lifters claim to actually press more doing declines than with regular flat benches.
Less distance means that more weight can be lifted.
Many lifters may want to work their upper and lower pecs, but don't have access to incline or decline benches. For working the upper pecs, I suggest a wide grip bench lowering the bar to a point high on the chest, towards the head. If the elbows are kept at right angles to the body, the chest is emphasized even more. This really puts stress on the upper pecs and I feel is more effective than inclines. Be careful when starting to use this exercise.