Is it actually confirmed that haste is calculated after talents? So on Greater Heal, this would mean that the casttime is first reduced by 5/5 Divine Fury from 3.0 to 2.5 and then haste is applied. The other way round would make haste quite a bit more effective.

I have a 2.25 cast chain heal. I've totally regeared for haste, I feel no need to stack mp5 or actively try to get it anymore since the water shield and mana spring buff. I never run out of mana except on illidari council where I spam my inefficient lhw. As far as I'm concered haste > all. if you run out of mana recruit more shadow priests or something(wtb more shadow priests btw)

I'll agree with most of what's been said already. Spell haste is pretty lackluster in its current iteration. Too expensive in item stat allocation for too little return.

But I'd also like to add, healing with some spell haste vs. healing without isn't something you can model very well. I don't believe there is a clear distinction such as x haste is equal to y +heal and z mp5 for everyone. There is a rhythm and timing to effective healing in a raid situation. Processing information like priority healing, will chain heal be more effective, should I be precasting on someone about to take damage, is my target about to catch another heal from someone else, do I need to move soon. Mileage will vary on how much spell haste "feels" right. Raid with 150+ spell haste for a few days, and then switch back and you'll see what I'm talking about.

From my perspective, spell haste "feels" like a better stat than it actually is. Getting that LHW off just few tenths of a second faster just "feels" more responsive than a standard cast time. I use it for RoS, Gorefiend, and trash usually. As a previous poster already stated, its a good stat, but not worth sacrificing others for.

It should apply after all talents. For example I currently have 72 haste rating on my warlock and Shadow Bolt shows as 2.39s on the tooltip. Base speed 3.0 seconds, the Bane talent brings it to a 2.5-second cast.

2.5s / (1 + 72/1577) = 2.39s which corresponds with the tooltip.

If spell haste applied before talents it would be
3.0s / (1 + 72/1577) - 0.5 = 2.37s

Edit: Used to apply before Light's Grace for paladins, seems that since 2.3 that was changed to match other talents as well.