Armour Penetration and you

After i got Grim Toll from Naxx last night, I thought i would read up on armour penetration and what it means to us hunters

Rough DPS Increase from Armor Penetration*

You need around 135 armor pen rating to get a 1% dps boost off of a raid boss (assuming sunder armor is up).

i made the mistake of disregarding armor pen as a pvp stat and ignored gear with this on however under heavy investigation ive learned that NO!!! hunters DO want Armour penetration and seen as you get alot of gear with this on you might as well use it.

For BM hunters - as BM shot rotaion is purely Serpent sting - SteadyShot - SS - SS and arcane shot added in dependant on the build. Armour pen affects your phyiscal attacks only. SS and AutoS are a BM hunters bread and butter shots and both shot dmg is increased with armour pen.
For MM and survival its not that essential as we have arc shot (improved), and aimed shot or Explosive shot all of which bypass armour anyway.
So dont ignore armour penetration. balance it with your other stats but dont nerf crit and agi for arm pen try to keep it in balance.
in short the more Armour Pentration the more dmg your SS and AS will do, so you think.....

However,
The bigger the warrior's Sunder Armour, the less effective our Armour Penetration is. The squishier the enemy -- i.e. the less armor he has -- the less armour-reduction-efficiency our Armour Penetration returns. Basically, the more Armour Penetration rating we hunters stack, the less value we get for our efforts. Stinkin' warriors...

Armour Penetration is not a bad stat, it's just the least-effective one we hunters have when we factor our party members interfering with our Armour Pen's efficiency. Adding Armour Penetration rating isn't a terrible thing, just remember to never sacrifice Agility or Ranged Attack Power or Crit to take Armour Pen.

Warriors use Sunder Amour to reduce a flat amount of armour. If a warrior is able to stack five Sunder Armours, he will reduce the enemy's armour by 3925
A raid mob with 13,000 armour for example. We know that the warrior's Sunder Armour reduces it by 3925, leaving 9075.

When the hunter attacked his own 13,000-armour mob, the 10% armour penetration reduced the armour by 1300. But when the hunter attacks the mob that the warrior is attacking, the armour penetration is calculated on the post-Sunder Armour value of 9075. Our Armour Penetration is going to reduce that value by 10% -- 907 -- leaving the mob with 8168 armour.

Well that's not bad, but it's certainly not what we wanted. We wanted 3925 from the warrior and 1300 from the hunter. But since Armour Penetration is calculated on the mob's acting-armour, we only reduced the armor by 907, not 1300. Our Armour Penetration rating, which we worked so hard to raise, has had its value reduced from 10% to just 6.98%, (907 / 13000 = 0.06979 or 6.98%).

here is a link about armour penetration in more depth.
http://www.warcrafthuntersunion.com/2009/01/hunter-armor-pen-rating-how-it-works-at-level-80/

All in all dont spec for armour penetration just use the gear you want some is likely to have armour penetratin on anywho.
As for the proc rate on grim troll been testing it today; the proc rate is fairly decent.