I don't have a Chaos army and I wouldn't really want one, but I have to admit that Chaos is a really cool side anyway. The concept of Chaos is really cool, since the Warp is the most powerful force out there. Ships getting torn apart by billions of billions of nightmare creatures... but I digress.
Being a Chaos Lord and leading tons of awesome-looking Chaos Marines into battle is hard to beat. It's gotta be awesome to paint up your Marines in brass and metal, giving them an awesome chaos-y look. Then don't forget about the coolness of being able to diss on the Emperor, pissing off any vanilla Space Marine players. Or you could just piss off the Eldar by mentioning that all their souls will be eaten be Slaanesh after the fight.
There's one thing more fun than being a Chaos Lord, and that's being a DAEMON PRINCE! Okay, the old models looked like garbage, so you'd have to convert one, but the new Khornate Daemon Prince looks totally awesome, and I hope they'll be realeasing more soon. Still, if you've got time to spare and the legendary Bits Box, you can try your hand at making an awesome Daemon Prince for yourself.
The Daemons look pretty cool too. The old Daemonettes looked extremely stupid, but I gotta admit that the new ones way exceeded my expectations. But my heart belongs to the Screamers of Tzeentch... those things totally ROCK! And the Nurglings deserve a mention, if for no other reason than the fact that the word "Nurgling" is so fun to say.
The Greater Daemons look pretty cool too, especially the Lord of Change. He's got that awesome bird-sorceror thing going on, and cashiers will love you because you'll always pay right down to the penny. Then when some hot chick is trying to pay for a coke at the coke machine, but she's only got fivers, it's His Lordship to the rescue!
The concept of the Thousand Sons is also pretty neat. Even though usually the psychic powers of Tzeentchians are just like guns, but less powerful, the Thousand Sons are bad on their own. Oh, and I sat up during Medieval Philosophy class when Ahriman was mentioned. He was the Zoroastrian god of evil. Now you know.
I have had the personal honor of flipping through the new Chaos codex, and I don't know about the rules, but the art in there was friggin' amazing. I almost wanted to renounce the False Emperor and embrace my true nature in the field of battle with a whirring chainaxe in my hand and a howling mob of Berserkers around me. Unfortunately I hear that the Chaos characters were seriously de-pantsed in the latest version, but oh well, you can't have it all.