Spell Haste

Recently, I was the sole healer in a heroic instance rep grind. The boss does a nasty AoE attack that takes the party down by half. I had to choose whether to use my more pricey group heals, or try to conserve mana while getting the most bang for my buck. Speed was important, but Flash Heal is also pricey, and should only be used in the most dire of situations... luckily, I had Spell Haste on my side.

Some will argue that using Ring of Flowing Life is not as great as Ring of Harmonic Beauty, since I lose out on Spirit and some additional healing. However, the spell haste and additional mana regeneration has saved many raids when it came down to the wire. I can quickly pull off a Heal or Greater Heal if need be in half the time of most priests in my raid.. and if a raid member or tank is toeing death, I can quickly bring them back from the brink.

Furthermore, in PvP situations, I can quickly take down an opponent by spamming Smite and still have the ability to stay alive with personal heals. Because of Spell Haste, my Smite (and other offensive spells) are very fast. I can cast it in almost half the time of most priests. If I combine it with PW:Shield and SW:Pain, I am deadly and difficult to take down.