Armor penetration gets a lot better as you get more of it, since it doesn't scale linearly. It's got a much lesser effect outside of raids, both because you can't count on full debuffs (Sunder, Faerie Fire, Curse of Recklessness) and because you can't get enough of it.
However, kitting yourself out in some major armor penetration is a pretty awesome damage increase. Take, for example, S3 helm, PvP badges cloak, leather chest from ZA, badges bracers, leather belt from ZA, badges or S3 legs, leather boots from MrT, badges ring, twin daggers from Hex Lord, and the bow from Zul'Jin. With that you're rocking about 1250 armor penetration without sacrificing *too* much in terms of other stats (main problem will be hit rating, and mana, but at that level your pallies should be judging wisdom and you should be chainchugging mana pots anywho), which is enough to completely inflate the value of armor penetration. I believe at that level it's more than double its value when you didn't have any.
EDIT: Hrm. I guess I with the set I just suggested you lose quite a bit of crit, I didn't think *that* much was lost, but comparing to one of my guild's hunter's (pretty much full ZA/T5/badges) it's about 10% crit difference, which is a lot. Stacking it might not be as good as I had though -_-
Edited, Apr 24th 2008 12:44pm by lsfreak