Such Penetration. Vel'Koz approves. Phreak too.
This is a chart i made which allows you to calculate the % increase in damage from buying Armor/MR penetration and even to compare 2 penetration builds.
Tl;dr: Look at this.
Ain't much for me to say. Try some stuff on your own.
I was suprised by how effective flat pen is, especially when coupled with Last Whisper/Void Staff.
Feedback appreciated. (I dont care if you hate Math. I like it.)
Find speeling mistakes, keep em'.