Spirit/Hit (until capped) >> Intellect > Haste = Mastery >> Crit
Important note for 4.0.6: the changes to overloads with this patch have brought up mastery to a point where it is equal to haste, and so generally speaking you do not want to stack one over the other. Keep that in mind when reforging or choosing upgrades.
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How should I gem?
As many Intellect gems as possible while getting good (+int) socket bonuses using int/haste orange gems or int/spirit purple gems. Your meta, like all ranged casters, is [Burning Shadowspirit Diamond].
Keep in mind that since you cannot reforge additional primary stats like Int, it is best to reforge to the hit cap rather than gemming for it, and squeeze as much Int as you can out of gemming (through gems themselves and/or socket bonuses).
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What stats do I reforge on my gear?
Follow these simple Rules for Reforging™!
If you're hit capped, and/or you have excess hit or spirit, do the following:
1. If an item has hit/spirit and crit, reforge hit/spirit to Haste or Mastery if you have excess hit rating, otherwise reforge crit to Haste or Mastery
2. If an item has hit/spirit and haste, reforge hit/spirit to Mastery if you have excess hit rating, otherwise leave it alone
3. If an item has hit/spirit and mastery, reforge hit/spirit to Haste if you have excess hit rating, otherwise leave it alone
4. If an item has crit and haste, reforge crit to Mastery
5. If an item has crit and mastery, reforge crit to Haste
6. If an item has haste and mastery, leave it alone!
If you're not hit capped from your gear, reforging crit to spirit can get you the rest of the way, as well as make it easier to exactly reach the cap with no wasted rating. Remember, however, to take gems and socket bonuses into account when reforging to the cap.
In other words, reforge crit to either haste or mastery whenever possible, depending on the other secondary stat on the item. If you have a choice between haste and mastery on an item, try to keep around the same amount of both stats.