Last Whisper becomes more effective at penetrating armor than The Brutalizer when an opponent has more than 42.8 armor. It becomes more effective than Youmuu's Ghostblade at 57 armor. It becomes more effective than 4 stacks of the Black Cleaver's debuff at 300 armor. It is to be noted though Last Whisper can penetrate more armor at those values, it is important to remember that the overall DPS may or may not be higher due to factors like runes and items stats.
This item has an AP counterpart which is the Void Staff.