Originally Posted by
Bzl
If you currently are on 13,3k spirit, and will fall down to around 11k, the transition might leave you a bit short changed. I would hold off until I got 2, maybe 3 items, and then make the transition, to ease the hit you'll take to your spirit.
The haste build reduces your GCD down towards the cap, and because you'll want to use qued casting, more spells will be used per fight, and more mana consumed, meaning it might be a bit more mana aggressive for you.
In either regard its worth trying so you know the differences between the two (haste vs mastery), but possibly something you'll want to try a week or two from now when your gear is more suitable.