The way that armor works is that it goes in the divisor. Imagine if you did 100 damage, and the opponent had 100 armor. The damage modifier provided by the armor would be 100 / (100 + Armor), or 100 / 200, or 50%. You would do 50 damage.

With 25 Armor Penetration your damage modifier would be 100 / 175, or ~57%. You would do 57 damage.

This looks even less impressive than +25 physical damage (which would be amazing), until you consider that Attack Damage marks give +.32 damage per mark, versus +1.96 armor penetration per mark.

Using OP's Xin Zhao as an example, Xin's base damage at level 1 is 55.3. Garen's base level 1 armor is 21.7. With no armor penetration Xin would do 45.4 damage to Garen, but with full armor penetration marks he does 53.1, or 7.7 extra damage per hit. Full attack damage marks are 2.88 damage.

Take a look at this thread:

http://www.leagueoflegends.com/board...=237269&page=4


It should gives you answer about how people feel about these runes.