Depends a lot on the situation, Mazra, but usually you're best putting on CoA or nothing at all.
If you take a look at Damage Meters as an Affliction lock, you'll see that in Midlength fights (say 20-30 seconds) your CoA will actually outdamage Corruption, UA, Immo, and SL. If you're in a situation where you're taking on groups (ergo DoTRot and not SBSpam) you'll actually find CoA doing upwards of 35% of your total damage ouput. Now add on the fact that it's one of the cheapest DoTs we have mana-wise and you have yourself a really nice DoT.
Big problem with it is that the early ticks of Agony are ridiculously low damage, so fights lasting under 10 seconds arn't worth the Mana, and fights between 10-20 seconds generally will even-out manawise with Corruption and make the GCD use worth it. The last 2 ticks are where Agony really hurts.
CoS increases all Shadow Damage by 10-13% but lasts a helluva lot longer. Problem here is it still busts a GCD, and you'll be needing to make up for lost damageoutput with SBSpam. If a Fights really going to last 5 Minutes (Bosses) then you're actually better tossing on Amped Dooms generally, UNLESS the situation calls for something different.
Examples:
1. Farming Level 62-68s
Personally figure out the most efficient way of doing damage, this usually includes having a DoTRot that finishes up just around the time the monster dies. You're looking at CoA not only being effective but being you're best friend as it is very low in the Mana department. Even with a Demo setup you should be Dotting up and taking on Multiples or you're wasting a lot of free time.
2. Doing Heroix with Feral Tank, Holy Pal, CRogue, Fire Mage, Demo Lock
This group is essentially a "everyone do your own thing in the same general area" sorta group - which is what most PUGs are. You should be focusing on doing the best at whatever it is you're assigned to do (Assist CC and DPS probobly.) The only danger with CoA is that like all DoTs it can interfere with CC but with proper Focusfire you should be doing CoA as the fights here should be just about the perfect length for CoA
3. Doing Kara with Fury War, CRogue, Feral, Demo Lock, Dest Lock, Fire Mage
Here the Locks are going to probobly round off the Top of the damage charts. You have a wide variety of damage inputs that are going to make using CoS/CoE ineffective (even 1 from each lock) and you're going to Find that on Bosses your best option is to just Doom (even the Dest Lock.) This is almost like "everyone do personal damage... at the same time."
4. Doing Kara with Marks Hunter, SPriest, Boomkin, Aff Lock, Dest Lock, Arcane Mage
Here you're going to see a really nice synergy with the DPS Classes that is going to make CoS + Doom an incredible combination and allow the Arcane Mage and Boomkin to really perform and the SPriest to land some Mana. This setup will outdps the previous by leaps and bounds simply because of their collaberated fighting style. Sticking the SPriest with the Boomkin, Dest Lock, and Cane Mage and adding in a Shammy for Hero/BLust will create burst like you've never seen.
5. Doing 25 Mans
Almost reguardless of the DPS setup, having a Debuff on the Boss that increases Arcane and Elemental Damage by 10% is so ridiculously powerful that there is no question. If there are any Malediction locks going along, people should determine who is doing what Curses before hand, but CoE and CoS NEED to be up on that boss. The rest of the Locks should be throwing on Doom.
Bottom line here is the effects of CoE/CoS are only truely effective when the DPS coming out of the *group* as a whole makes the 10% worth it. If nobody else is taking advantage of the damage increase, it's very likely that Agony or Doom would suit the situation better. Remember that some fights aren't worth busting the GCD at all.
Edited, Mar 24th 2008 1:41pm by tzsjynx