In the level 30 range Imp CoA is actually not a bad choice for Affliction Lock. It is one of your few Instant cast DoTs, and it's used quite often.
Later down the road, though, there are far too many alternative options that do a better job. Curse of Doom does effectively more DPS, DPMana, and requires fewer casts which is less time lost due to GCDs. In larger settings, casting Shadows/Elements is another alternative that can really help up overall DPS moreso than even Doom can.
In PVP Doom is not such a great idea (if the point is to not only kill them but do it before they kill you), so on select targets Agony is still used. But in many cases a better choice can be made: either Exhaustion for melee or Tongues for casters, or again Shadows/Elements if you plan on burning them down with Sbolt/burn or Incinerate.
There are just too many alternatives when you get higher level. I would totally agree with MrDallas in the fact that for lower levels where Emp Corruption doesn't help damage since you hardly have any +Spell, ICoA is not a bad choice. Once you get a fair amount of +Spell, though, and you get CoD, it's time to respec.