To Ascero,

Great call to clarify the difference between rating and %. However....

"As such, I take that to mean that it will actually decrease your casting times of spells by 12%."

That's not quite how spell haste works. Each 1% haste allows one more cast in the time it would normally take to cast 100 of the spell.

Instead of 3sec becoming 2.64 with 12% [3 - (3*.12)] it really is 2.68sec* [3/1.12]** It sounds trivial, but they are different. The way you phrased it while incorrect is a reasonable approximation. Works fine for eyeballing gear but breaks down as more and more haste piles on. The ratio changes when you graph them against haste and cast times.

The GCD reduction, which is ultimately more important, is different math entirely. No idea what the equation is. Also, haste rating is additive (rating + rating = rating that converts to %) but haste % is multiplicative (% * % = %)***.

* No idea where Blizz rounds. 2.67857142857repeating.

**That's 3/(1+(haste/100)) to expand the equation.

***10% from one source and 10% from another = 12.1% haste, not 20%. Or, 327.9 rating + 327.9 rating [32.79 = 1% spellhaste at 80] = 10% * 10% = 12.1% haste. That's not as ugly as it may look. Always up 1% haste can generally be thought of as +1% DPS for casters like mages whose DPS isn't heavily reliant on DoTs or instants until the 1 sec GCD is hit on the nuke(s). It scales very well.

Sweet baby Thrall that's a lot of footnotes! Hope I didn't confuse people even more. If there are any typos in the math, someone please correct them.