Simple question, really. How many points is good luck worth? In a lot of ways it's a question you have to answer already, as you can choose to spend extra points for reliability (adding a Combi-Weapon to a squad for some extra oomph, master-crafting or otherwise buying rerolls, etc.) However, Blood Angels are in a unique situation, because they can buy luck directly. What do I mean? Enter Corbulo, Sanguinary High Priest
Corbulo doesn't just buy luck, of course- he also comes with a spare wound, a S5 Rending sword, and passes FNP tests on a 2+ while otherwise having everything that a basic Priest comes with. But if you're taking him you're taking him for that once-per-game reroll. Is it worth it? That depends.
One thing to consider is that it is a flat reroll, a do-over. You can't use it on the roll to go first, but any other roll in the game that you make is fair target. Since his ability lets you re-do a single roll, it applies to all dice rolled as part of a single action- so, for example, you would reroll an entire unit's shooting attacks or saves at once, all of the attacks from one unit during a single initiative step, all of your penetration or vehicle damage results from one of the above, etc. Note, however, that you reroll ALL of them, not just the ones you dislike. Also remember that you can never reroll something more than once, so if you have master-craft weapons, a Chaplain/Reclusiarch, etc, there will be times you cannot take advantage of him.
So is he worth the 55pts you pay over a normal Priest? Well, were you giving that Priest any special gear? Many lists simply don't have the option to realistically bring him, as they need to put Jump Packs on their dudes (or don't need a Priest in the first place.) However, if you are bringing Priests on foot, he is a pretty decent deal; against anything except ID attacks, he is actually more survivable than a Priest in Terminator armor, so he ends up being quite difficult to bring down. He also gets an okay CC weapon, although not as good as the Power Weapon that you might otherwise be handing to your Priest. He gets an extra attack in compensation, though.
But again, are you making use of that reroll? Some armies really can use it; others not so much. Everyone knows that the whole game can easily boil down to the roll of a single die, or perhaps a few dice. If you can keep Corbulo alive, he can provide a critical advantage at that juncture. For armies with Stormravens or Land Raiders, this can be a big deal, as they make getting past that critical T1/2 juncture a lot easier- remember, though, you can only reroll your OWN dice, so you need to be passing those cover saves. Once the opponent has moved on to rolling damage results, it's too late to use his ability. Here again it is important to remember that you reroll ALL the dice, no matter whether they were passed or failed, so if you pass two out of four Flat Out saves, you probably don't want to use him. Terminator armies can also make good use of him- I began considering him after some recent experiences with rolling multiple '1's on poor rounds of saves, and was looking for a way to mitigate that. Any time your game can be heavily reliant on one of a small number of rolls, Corbulo can help.
The real trick, though, is figuring out when to use him- and for this there is no easy rule. You only get one reroll, and it must be used while he is alive. Will you retry those Melta shots to break a tank? The armor saves to keep your Marines alive? The mediocre hit roll when you assault? It can be easy to just keep putting it off waiting for something better, but as a general guideline you will want to use it on any unusually bad roll that you were counting on succeeding. Derp, I know, but do you know the odds well enough to say what is "unusually bad"? You rolled four hits out of fourteen dice when you charged in- is that bad enough to warrant using it? Understanding the approximate odds is critical to making good use of Corbulo's ability.
An Aside on Wording
Now, all of this hinges on one big assumption of mine: that Corbulo allows you to reroll multiple dice at once. However, I think this belief is relatively well-supported by the rules and wording of his ability. Let's take a look at it for a second:
"Corbulo allows you a single reroll once per game. This can be of any roll you have made, such as for an armor save, a Leadership test, a To Hit roll, a scatter roll, or even to seize the initiative."
Why do I believe this allows you to reroll multiple dice? For one, GW's terminology uses "reroll" to mean all of the dice for a single event or action, such as a unit's shooting or close combat attacks. (Check the text on such abilities as Preferred Enemy for an example of such language.) To contrast, in cases where only a single die is rerolled, the phrase "reroll a single dice (sic)" is universally used, as with Master-Crafted weapons and Digital Weapons. It might also be argued that only the dice for a single model are rerolled- however, I also believe this to be incorrect, as units are universally rolled as a group, even in descriptions in the rulebook, which constitute a single "action" within the context of the rules. (For melee attacks, of course, this applies only to the attacks made during a particular initiative step by a particular unit.)
