Originally Posted by TrevvyTrev

That's what makes it a "not so simple" question. Whether Armor Penetration is good enough to trade for depends entirely on what you're trading. As a general (but not ironclad) rule, it's better than haste is for hunters, but whether it's better than crit or hit or AP or AGI depends on many other factors, not the least of which is what your other stats already are. It's not even as simple as it's worth it for one spec and not for another. The closest you can say is that BM gets the most out of it (because more of their shot damage is physical) and survival probably gets the least out of it.

This is one of those "plug it into the spreadsheet" questions, unfortunately.