Although I am sure I have had them before, I actually SAW my first Experience Bomb last night.
I had just killed one of the undead that pop up out of the ground in the Black Ring Dungeon, and looked closely to read "Experience Bomb" in the tiny font next to the dying corpse.
I also saw the reward that has previously been pointed out for the Experience Bomb: Ten times the experience (48,000 exp. for a 4,800 exp. monster)
As I adventured on I noticed something very interesting: Where it had always taken 2 or 3 hits to kill this type of monster before, now I was killing them with only 1 blow. This happened again and again (although I had not leveled up or changed anything about my armor or weapon)
So, I hypothesize that the Experience Bomb has a second reward that allows you to inflict more damage on that type of monster in the future.