http://www.tankspot.com/forums/f207/60212-too-much-haste-resto-shammy.htmlAn interesting thread discussing Haste vs. SP vs. Crit vs. Mp5 as it relates to resto shaman gear choices, and more importantly, epic gem choices.
I am of the mindset that haste (up to 1036 rating or ~30%) is the best stat to stack for overall raid performance and throughput. Your only other consideration should be for Mp5, and even then, only if you're constantly going OOM and causing wipes. Your best option is to find the best mana regen trinkets you can get so that you can mostly ignore Mp5 in almost all other slots. With raid buffs, Mana Spring/Tide Totems, water shield, replenishment, and an emergency Mana Pot or Innervate on longer fights, you should be fine.
SP vs. Haste Only two stats reliably boost your throughput: SP and Haste. Every piece of gear besides your trinkets is going to have a fair amount of SP on it. The same cannot be said for Haste, which is partly why I gem for it. I think that certain epeen tard players want to be able to spam "ZOMG 3500+ Heals Shammy LF Hardest Raid Content!!!" in chat. While that works for a mage (Arty!), you have to factor in the concept of Overhealing as a Healer. A DPS class cannot Overkill until the boss is dead. This happens once every 5-10 minutes on boss fights, and thus deserves no consideration for PvE. This concept is ironically in part why a lot of PvP casters DO stack haste - less overkill, more effective damage to multiple targets amongst other benefits.
So as a healer, since most spells have some component of overhealing, SP is going to contribute heavily to your overhealing, but not necessarily as much to your effective healing. I prefer the healing style of being proactive, and keeping the raid topped off with small quick heals, rather than relying on my Spell Power to bail me out when I fall behind due to slower cast times or poor reaction time. Especially as a class with only one fairly weak HoT, and almost all direct healing spells, haste just makes sense.
I'm sure that there are certain situations where spellpower is better. Maybe pure tank healing - where you need spams just to keep them alive is one possibility. Even then, there is probably a minimum value of SP that you have to reach to overtake the benefits of haste. You then have to consider how often you're going to be assigned to heal the main tank as a Shammy. With Kyndness and Bubble making every major raid, I'm really not too worried about this in the near future.
One last note - I typically choose Crit over Mp5. Crit is a hybrid throughput/regen stat for shammies. Stacking Haste and crit has a lot of synergy with the Improved Water Shield talent. Crit can also benefit your chain heal as much as spellpower since each link can crit, and it's a diminishing smart heal. I would never, ever gem for crit or seek to stack it on my gear since it is unreliable. However I think it gives a greater benefit, per item point allotted than Mp5. Blizzard attempted to correct this by boosting Mp5 on ALL items in the game not too long ago, and probably will have to again soon since Resto shammies basically roll Ele/Dps caster nowadays.