Mage Question

[Xanthix]

In order to answer this properly, you need to understand the mechanics of spellhit and spell penetration.

The game labels all spells that do not have their intended effect as "resists", regardless of whether or not this is actually true. Just as with melee attacks, there is a miss chance with spells. Spellhit counters this (up to a cap). Actual resistances are different. X spell penetration essentially removes X resistance from the target.

This means that spellhit is always useful, but there is a cap on it, and spell penetration is only useful if the mob has actual resistances. I believe the cap is 4% for mobs of your own level, and 17% for bosses, though don't quote me on that.

Those numbers look right, for PvE. Here is the official reference (look under "Items: +Spell Hit Chance"): http://www.worldofwarcraft.com/info/basics/spells.html

I can't remember if there is a minimum 1% resist rate, in which case you actually aim for 3%/16% to hit instead of 4%/17%.

And as far as I know, no one really likes spell penetration gear. If you have the relevant curse it's completely moot, and it has even been shown to have no effect on the +hit or duration of polymorph/shackle undead etc.