I haven't seen a thread dedicated to this addition to itemization traits, so here we go...

There are more than a few items on PTR (one, I think discovered on Live) that have an armor penetration trait. There will most likely be more uncovered or added as time passes. While haste rating additions are already well understood and easily implemented into the spreadsheets, Armor Penetration has not been explored very much. Primarily I think it's because individual Armor Penetration stats have not really be worth much note (see the Warp-Spring Coil thread). But now that there is more of this stuff around, combined with the fact that the more you have, the more you get out of it, I think it's worth looking into.

I guess my first question is: is it worth it? Obviously it does more on targets with lower mitigation, but at what point would this stat be outweighed in terms of item budget vs dps gain due to high mitigation mobs? The more you have, the more it's worth and it's valued differently against different mitigation levels and even then that depends on the amount you currently have available to you.

Will melee DPS need to now consider carrying two sets of gear around to account for this? This would wreak havoc on certain distribution systems.

Bringing these items to bear on soft targets in PvP is a scary thought and I think Rogues in particular will suffer greatly on the receiving end, and all classes with access to this stat will have a field day against low mitigation classes.

Seems like a headache trying to work out equivalency points since they work off of what you already have, like I mentioned. Maybe someone has an elegant way to work this in.

Are you sure this is true?

Armor doesn't have diminishing returns in the traditional manner. Every X amount of armor you remove reduces the bosses time of life by Y duration. If you took an enemy that takes 100 seconds of stabbing to kill, you could use an item that ignores 300 armor and this would reduce his lifespan by the same set duration regardless of whether it had 5000 armor or 20,000 armor.

Actually if someone could toss up the real AC formula (I've been honestly looking for it) we could hack together a quick table of how much this increases your dps. Something like:

1000 AC == 10% extra dps (I actually think it's somewhere close to this)
600 AC ==5% extra dps

ect.

Of course it's deceptive because you cannot simply add the numbers together, but I know I'd find it pretty handy to look at.

Also the next time someone tells me AC dimishes I could beat them over the head with this table carved into lead, so that's a bonus.

edit:
TO clarify, yes you're right BEEF to my understanding, but you can also look at it as 'I am hitting for 100, mob loses 1000 AC, I'm hitting for 110 now'. In terms of white charecter sheet relative increase you do gain the biggest #'s on lower AC, but it's always going to be a static % as relative to how much you're actually hitting the mob for.

This is the formula I have from a while ago. It seems to be accurate compared to the tooltip anyway.

% Reduction = Armor / ( Armor + 467.5 * <Level> - 22168)