I got to be pretty good with my Undivided Chaos army over the past few years. Since January 1, 2004 I have amassed 23 victories, 16 losses, and 12 ties which is the mark of a good player who at the same time is not a power-player.
I built those victories off pretty much the same core units:
1 x 10 Marine Assault Squad including meltagun and plasma pistol with power fisted champion in rhino
2 x 20 Marine Tactical Squads with lascannon, plasma gun, champion with power sword, sometimes in rhinos
1 x 10 Marine Tactical Squad with Missile Launcher, plasma gun, Infiltrate, champion with power fist
Everything else was built around that core of about 1000 pts. The reasons for why this core was so important can be contained in one word: flexibility. Except for the assault squad, these units could be assigned any duty with relative comfort. Shooting infantry? Bolters and plasma gun would take on anything. Anti-tank? Heavy weapon and plasma gun were good combination. Close quarters? The champion gave the squad the power it needed to stand up to many threats. Holding objectives? High re-rollable leadership and large squad size made them difficult to shift. Even the assault squad was capable of going after vehicles or infantry with ease. And combined they proved durable for the army; if I lost one squad early on, I still had three to fall back on.
This playing style suits me to a tee. It allows me to move with some confidence since every unit is built to deal with the same situations. The rest of the army is in support of this core and many games of practice have given me sure footing with it. Which is, of course, the reason I'm changing it all around.
Playing Epic for the past year has reinforced the concept of dedicated roles to me. Having a unit that needs to sit still to shoot for some models and move and assault for others is a sure way to neuter the entire unit. In 40K, chaos units have the option of taking 1 heavy and 1 special weapon, OR 2 special weapons. I've begun playing around with the second option in my tactical squads in an effort to make them more mobile so that I can advance with a bit more conviction and still get heavy weapon support from havoc squads.
The experimenting is ongoing, but so far in a couple games I have been moderately pleased. We'll see if it lasts.