The usefulness of this talent will depend a lot on your raid composition and the role you play as a result of that composition. If you do most of the tank healing in your raid team (maybe you're a 10-man raider with a resto druid as your healing partner) then this might be an ideal talent for you. To know for sure though, I recommend paying attention to your logs for a couple of weeks and add up how many free Flash Heals you're getting. If the mana you're saving is greater or equal to what you'd get from an alternative talent like Mindbender, then you've made a solid choice. If it's significantly below though, you should switch.
Surge of Darkness For shadow priests, From Darkness, Comes Light becomes the very desirable talent: Surge of Darkness.
Mind Spike is usually a very situational spell; Surge of Darkness changes that. By removing the cast time, increasing the damage, and getting rid of the mechanic which consumes our DoTs, Mind Spike becomes a very strong addition to our rotation which fills in some of the awkward gaps (where you'd otherwise Mind Flay) nicely. In priority, Mind Spike procs will come after Mind Blast and Shadow Word: Death, and before DoT refreshes, provided you use two procs in sequence (you don't want to inject just one proc at a time -- wait for two).
Surge of Darkness will work best for a shadow priest when used on an encounter with multiple adds, since the talent procs off Vampiric Touch ... The more targets afflicted with Vampiric Touch, the more chances you have for Mind Spike procs. (Tip: Pairing this talent with the Glyph of Mind Spike also grants you addition utility, by reducing the cast time of Mind Blast when you cast it within 6 seconds of using Mind Spike.)
Your other level 45 talents
Mindbender By replacing Shadowfiend with Mindbender, you are given a spell that deals less damage and returns less mana per cast than Shadowfiend, but ultimately provides more of both when used on cooldown over the span of the fight.
So how much mana does this little guy give us? Currently Mindbender is returning 1.43% of your total mana per Mana Leech, which amounts to 4,380 mana with our static 300,000 mana pools. How many leeches you get will vary on your haste. A player with no haste will get 10 leeches by default, but you're more likely to get 11 (or even 12) with some gear. So count on about 48,000 mana returned per cast. For comparison, Shadowfiend returns more than double that amounds per Mana Leech (3% of your 300,000 mana pool) but you'll only get 8 leeches with no haste. With some gear though, you can get 9 leeches, so you're looking at 81,000 mana per cast.
What about the damage? Well, Mindbender swings do 80% of the damage a Shadowfiend's swing does, but since the spell hits one to two times more per cast, and can be cast once per minute, you're looking at roughly three times the damage from Mindbender in the time you'd get one cast from Shadowfiend. So at about 20,000 spellpower, you can expect roughly 220-242k damage per Mindbender and 218-244k from Shadowfiend. Consider the difference in cooldowns and you'd need to use four Surge of Darkness procs per minute (plus Shadowfiend on cooldown) just to match the damage Mindbender does. If you can do more than four, like six or eight (remember you want to do them in pairs), then Surge of Darkness is going to win out. This is why Mindbender is better on fights with a single target, since you have less targets to proc Surge of Darkness off of.
Power Word: Solace This talent puts you more in control of your regen by letting you restore mana whenever you cast it. Each cast only returns a small amount (currently 0.7% of your total mana, which is 2,100 mana) but if you have enough downtime during a boss fight, you can get more mana back than you would with Mindbender. So how much do you need to cast? Well, if you use Shadowfiend early (within the first minute) and then use it on cooldown after that, you'd want to cast Power Word: Solace about 10 times per minute to match the mana Mindbender brings back.
Whoa ... 10 times? Yeah, it's a lot, but consider this. The amount of damage you see in a fight varies from second to second. You'll often have phases where you need a whole lot of mana, then another phase where you almost need none. Mindbender is very efficient over the span of a fight, but it's only going to ration out a little bit of mana to you at a time. It's like the difference between taking lottery winnings in one lump sum versus small monthly or annual payouts. Taking the lump sum gives you more money at once, allowing you to do something big immediately after; it's the same with mana. If you use 20 seconds of downtime to cast Power Word: Solace 7-10 times, and use Shadowfiend at the same time you're going to have over 100,000 mana for the next huge wave of damage. Mindbender limits you though, so even if you get more mana over the span of a fight, you might not have all the mana you want at certain points in time. Mind you, the same can be said about Power Word: Solace if you don't maximize your opportunities to use it, so it can really go either way. Experiment with both from fight to fight, and find out which one works best.
Shadow Word: Insanity As for shadow priests, Power Word: Solace becomes Shadow Word: Insanity, and it's currently a very undesirable talent. The window of time you have to use the proc from this talent is quite small, and since it doesn't take priority over some of the spells in our rotation, it's easy to miss an opportunity to use it if a higher DPS spell becomes available at the same time. At some point I imagine a fix is coming, but it's yet to happen.
Level 60, self-preservation
Desperate Prayer With this talent you can heal yourself every two minutes for 30% of your total health. The spell is instant cast, costs no mana, and will not trigger the GCD when used (it can also be used while in the GCD). Keep in mind, it will not interrupt a cast you're in the middle of, so if you want to ensure this spell is used when you activate it, macro it to a stop cast command.
Now since Spectral Guise is a very situational spell outside of PvP, most players are going to be looking between this talent and Angelic Bulwark. Why pick one or the other? The advantage of Desperate Prayer over Angelic Bulwark is that you have absolute control over it. Every situation where your health drops below 30% is not necessarily a time when you need immediate healing or additional safeguards, but Angelic Bulwark would activate regardless. This means when you actually need it, Angelic Bulwark may already be on cooldown.
Of course, being in control of something has its own disadvantages. If you lose control of your character, you won't be able to cast Desperate Prayer (Angelic Bulwark on the other hand, will activate when you fall below 30% health, regardless of whether you're stunned, feared, or silenced). You also have to make sure that you use it ... If it gets forgotten in the fray of battle, then it goes to waste. (Tip: Desperate Prayer is a quick way to rebound from Void Shift.)
Spectral Guise This talent allows you to temporarily transfer threat to a Spectral Guise, thus diverting imminent attacks from yourself while you stealthily step away. There's no question that this spell is game changing in PvP (since an extra two seconds can make the difference between life and death) but in PvE practical uses are limited.
In a party Spectral Guise can function as a threat drop when you have aggro, but Fade provides a similar function without triggering the GCD. Fade also has a longer duration, allowing your tank more time to establish threat on the enemy target. (Plus the duration of Fade can't be shortened as Spectral Guise would be if struck three times by an attacker.)
For solo questing, Spectral Guise might be a good way to put space between you and mobs, but Void Tendrils works just as well. Obviously there are differences ... With Void Tendrils enemies will still target you and hit you with ranged attacks if they can, while with Spectral Guise enemies would immediately switch targets to the guise. The guise won't last as long as the tendrils though, since mobs aren't smart enough to target the tendrils.
Now, it may be possible to dodge some targeted boss abilities with Spectral Guise, but a player would need to test it against each individual mechanic to know when it was worth taking and when it wasn't.
Angelic Bulwark This passive talent automatically shields for 20% of your total health, whenever your health drops below 30%, though it can only occur every 90 seconds.
The main advantage of Angelic Bulwark (which should be somewhat apparent if you read the section on Desperate Prayer) is that it works automatically. You don't have to remember it, and you don't have to find a free second to work it into your rotation. It will also work if you're stunned, feared, silenced, or mind controlled. The 90-second cooldown is also shorter than that of Desperate Prayer, so you can get more up time out from it as well.
Disadvantages are that it may sometimes be wasted. There are several boss abilities in the game which will strike your group for a huge single hit of damage, but not be followed up with more damage to finish you off. These types of abilities are designed to punish players at low health, and stress out healers, but will rarely come close to killing you. Your Angelic Bulwark cooldown won't know the difference though.
There is also a slight disadvantage to receiving a shield over a heal, namely that the shield will eventually fall off, while the heal is only undone by taking more damage. With Angelic Bulwark, healers will still have to spend mana to heal you, even if they are bought more time to heal you by the shield (granted, the average healer will not even know they have extra time, since few of them will be aware you have an Angelic Bulwark on you to begin with). The impact overall is subtle, since group damage usually comes in waves ... Meaning by the time that shield fell off or was consumed, there might not be a need for its protection anyway.
Wrapping up next week
Next time we'll finish our look at priest talents, with level 75 and 90 talents. If you have anything to add to today's column (maybe an example of how you can use Spectral Guise in PvE?) please leave a comment.
[For level 15 and 30 talents, please see Choosing the best priest talents for you, pt. 1.]
Spiritual Guidance is WoW Insider's source for priest information. For information on playing the class, check out our Mists of Pandaria guides for discipline, holy, and shadow. For getting geared up, check out our pre-raid gear guide.