Finisher Efficiency: Eviscerate

Eviscerate is the only direct damage finisher rogues have. The damage it does depends on the amount of combo points used. It is mitigated by armor and it can crit, but it's independant from your weapons and attackpower. In other words: it doesn't really scale with gear. Evi might have been a no-brainer when you were leveling, but with Backstabs and Sinister Strikes yielding more and more damage per energy point, and white melee hits making up an increasing share of your total damage, the good old Eviscerate seems to drag behind.
To decide right when to spend combo points how, let's take closer look on Eviscerate's efficiency.

The efficency of Eviscerate with different amounts of combopoints

Without talents this is what an Eviscerate does on a target with 0 armor:

1 CP Evi does 216-312 dmg = 264 average dmg = 28%
2 CP Evi does 384-480 dmg = 432 average dmg = 46%
3 CP Evi does 552-648 dmg = 600 average dmg = 64%
4 CP Evi does 720-816 dmg = 768 average dmg = 82%
5 CP Evi does 888-984 dmg = 936 average dmg = 100%

The damage increases proportionally to the amount of combo points invested. You could say an Eviscerate does 94 base damage and 170 damage per combo point.

An Eviscerate will cost a certain amount of energy which could be spent to generate CP and damage, and with Ruthlessness specced the Eviscerate will be cheaper the more CP you have to spend. So obviously the 5cp Eviscerate is the most efficient one.

For lvl 60 rogues with decent gear, Ruthlessness and Relentness Strikes, Eviscerate will generally start to pay off with 3 or more combopoints. If you do Eviscerates with only one or two combopoints, you lose so much energy that it negates all damage Eviscerate may yield.
While it starts to pay off with 3cp the efficiency gain from using more combopoints is quite large. For example for a Sealfate Rogue in epic gear Eviscerates with 4 CP will mean a 60% higher dps buff than Eviscerates with 3 CP. Full 5 CP Eviscerates are twice as much extra dps as 3 CP Eviscerates.

Eviscerates and Sealfate

So the best way to spend your CP on Eviscerate is with 5 points. However with a Sealfate build you might waste CP if you go for 5 points at all costs: If the last SS or Backstab you do crits, then you lose the extra CP Sealfate is awarding for crits. So what should you do if you have Sealfate and got 4 CP on a target?
For a Sealfate rogue using daggers doing 4 CP Eviscerates or risk wasting an extra CP to do a full Eviscerate doesn't make a difference. If you use Swords/Maces, you are better off going for 5cp as the risk to waste a combopoint is a lot lower with Sinister Strikes.

Short fights

In short fights where you are fighting only one (or the last) mob it's not always possible to wait until you can do a 5 point Eviscerate. If the mob dies too fast you might lose all your CP. Even if you managed to get the finisher off, you might not have time to spend the 25 Energy from Ruthlessness. In these situations 3-4cp Eviscerates before doing a last Backstab or Sinister Strike can be a good option.

Example:
Let's say you are SF dagger specced and have 110 energy. Your raid engages an Anubisath Eradicator. These fights take about 10 seconds, so the best way to start is with two Backstabs and if they both crit, you do an Eviscerate. That way you get a 80% chance to get 25 energy back which allows you to do a 3rd Backstab right after you get the 4th energy tick. If you had tried to do the 3 Backstabs first and then finish with an Eviscerate, you'd have needed 2 more energy ticks (or 4 seconds) which you won't get.

Conclusion

Generally go for 5 CP Eviscerates if possible.

4 CP and 3 CP Eviscerates should be only used if you don't have time to get more cp on a target. They are noticeably less efficient than 5 CP Eviscerates but still better than wasting the combo points.

Eviscerates with less than 3 CP shouldn't be used at all except if you are sure that the Eviscerate will kill the mob and you have time to regen to full energy before you engage the next one. This happens sometimes when grinding.

In situations where you will regen your full energy until you engage the next target, it might be better to use Eviscerate (and other finishers) not as the last spell. That way you get a chance to spend the 25 energy Ruthlessness might give you back.