As to what effect percussion shells had when used against infantry - Lt. Benjamin F. Rittenhouse, commanding Hazlett's Battery on the third day of Gettysburg, wrote of the effect percussion shells had on Pickett's Charge: "Many times a single percussion shell would cut out several files, and then explode in their ranks. Several times almost a company would disappear, as the shell would rip from right to left."
In Rufus Dawes' "Service with the Sixth Wisconsin," when describing the opening actions of the battle of Antietam (page 87), he writes, "We had marched ten rods, when whiz-z-z! bang! burst a shell over our heads; then another; then a percussion shell struck and exploded in the very center of the moving mass of men. It killed two men and wounded eleven. It tore off Captain David K. Noyes's foot and cut off both arms of a man in his company. This dreadful scene occurred within a few feet of where I was riding and before my eyes."
I would imagine that the nature of the ground itself, i.e. soft and muddy, hard and dry, etc. would probably have an effect on whether or not they buried themselves in the ground prior to detonating. When percussion shells did work as intended it sounds like they certainly caused a lot of damage though.