Spell Focus: Prayer of Healing
Description
Prayer of Healing (Rank 6)
1255 Mana
3 sec cast
A powerful prayer heals party members within 30 yards for 1246 to 1316.
The Prayer of Healing is a very potent multi-target healing spell that is available to every priest. It costs a good chunk of mana, but if there is a lot of damage going around, this can be the most efficient spell to use.
Strengths
To be honest, Prayer of Healing has not many strong points. In fact there is only one in my book. It heals for a lot of health if enough targets are affected. So, if you group needs some health, but is not prone to the risk of perishing in the next few seconds… use this spell. After ensuring they are in range, of course.
Weakness
Where to start… First, it’s a rather slow spell with a 3 second cast time. If you can predict the incoming damage to be not spiky for the next couple of seconds, you can use this spell, else don’t bother. Also, it costs quite a lot of mana.
Additionally, be aware of the spell’s limitations, it will only heal your group, and only if they are within 30 yards. And remember, if you manage to use this spell to it’s full potential, healing 4 to 5 people for a significant amount of life, you will acquire a nice chunk of aggro. Be ready to Fade after casting a Prayer of Healing. 
When & how to use
Whenever your group is taking sizeable damage, and you are not tasked to heal anything critical, and you are able to stand sill for a few seconds, and your mana is not low, then, you may cast a Prayer of Healing.
When not to use
If you don’t know the fight, if you may need to move suddenly, if you cannot spare a few seconds, if your group is too spaced up, if only a few group members are missing health, if you need to heal a critical target, … and so on. Then do not cast a Prayer of Healing.
it’s really sad that set item bonuses affect this spell (such as 2 x T4, or 2 x T6). You rarely have the time and mana to cast this, when you have Circle of Healing to cast on the fly, with roughly the same results. I did find it useful a couple of times, such as on Gurtogg Bloodboil when being part of one of the groups that took the BB, or on Illidan Stormrage in Phase 2, when my group gets hit by a fireball.