The Possessed get a lot of stick for there points cost but if they manage to get Furious charge, fleet of foot, feel no pain or power weapon with there Daemonkin rule they can be a very effective unit, so that's a 2/3 chance of getting a decent power. Give them the mark of chaos glory as its so cheap but I have no idea why they don't have the fearless special rule?! But don't waste any additional points giving them any other wargear.
If this a super competative list you are best getting rid of them as there cost per model is over priced.
For a Chaos army theres not much overly inpressive CC units ... a chaos lord with a daemon weapon instead of lighting claws could help you out, I'm not going to tell you to get a Daemon Prince as thats always mentioned in these forums.
They can take the IocG... but its really more of a personal icon like the HQs can take. And for the same points. Still messed up really...
I agree with everyone really, possessed are fun but you could do better for competitive play. If you lost the havocs from your preds, gave your defiler extra arms, and lost the lord, you have plenty of points to buff up. Maybe add 2 bodies and a pair of melta guns to your csm squads. And perhaps a pair of winged princes with a mark of slaanesh?
I bring the possessed because they have done well in all my games, and the models/full behind them is pretty cool. I also have somewhat of a soft spot for units that are disliked. And the randomness factor can also screw with your opponent. The only time my possessed have done bad is when I have them try and fight CC specialists, but that's true for berserkers, too.
I've changed the list some after my last battle.
The chosen are now one 10 man squad with 4 plasma.
I have a dread with a plasma cannon/CCW.
The lord became a lash sorcerer.
Havoc launchers on the possessed/zerkers rhino's
I have the dread advance with the possessed/zerkers to provide some extra fire power.
The list isn't a tournament list, probably should have said.