No one seems to like lethargy anymore because it was turned down from GOOD to BAD damage dealer. Funcom took out 3 out of 4 combo steps leaving poor lethargy as one-step combo with +20.3 dmg on opener. Some people went as far as taking the lethargy out of their combo bar.
I would have to agree that Lethargy lost all uses in pvp: the damage is way too low and the debuff not worth the hassle - landing simple GC will do you better in every way.
However leathargy still is somewhat useful in PVE encounters:


Raid:
I don't have any experience at raiding, but from what I have gathered lethargy should always be droped on the boss to let the tank and a healer breath a little. Debuff giving -6% dmg may not seem like much but those things add up and over a long fight that debuff can translate into a lot damage being avoided.

Mobs: (see the youtube video - <a href="http://www.youtube.com/watch?v=uIjv0UJJr7Y" target=_blank>http://www.youtube.com/watch?v=uIjv0UJJr7Y</a>)
When you are fighting +2 mobs lethargy can help you stay alive by debuffing several NPC at once with cone effect.

As you can see lethargy affected 3 mobs so it was as your lethargy affected only one NPC, but for -18% dmg.

You can debuff more than 3 mobs with this litte trick: use lethargy to get the first hit (combo opener) and then dubble tap backwards (default "s") which will make the NPC try to get closer to you and "stack" (being closely packed).