My chaos army laid the hurt on Jay's tau last night when we played a 2000 point gamma take and hold match on my home table. The noisemarines and plaguemarines are far from finished, and I had to sub in a few other things too, but he was also using some proxies for a few of his battle tanks and suits. Here's his page. I took his main tanks down one by one with obliterators while the demon princes advanced on the left flank and the cult troops castled up in the middle, eventually joined by the terminators. The demon princes and plaguemarines absorbed just a ton of fire, while my own shooting wiped out a squad of battlesuits, several fire warrior squads, and some pathfinders. Most of my assault units never got to wreak too much havoc, and I only got one ineffective lash on the first turn, but otherwise chaos was doing much more killing than the tau and we called it in the sixth turn. I love cult troops! Specifically in this game, the noise marines' doom siren, the plaguemarines feel no pain (made a ton of those rolls), the thousand sons' inferno bolts, and the berzerker plasma pistols. Time will tell how great the build really is; it still seems more of a hobby army than a 'eavy one. I haven't found an old-style keeper of secrets model yet, or the chaos possessed wings for him.
I got the Demons codex - not planning to build an army any time soon, but I think eventually it will be just the ticket for my long-dreamed-of überproject: nurgle-infected orks. I'll probably include the named greater demon, Epidimius and a second nurgle herald, a bunch of orkish plaguebearers, a bunch of nurgling bases, a couple of soulgrinders (from ork dreads), and a flying nurgle demon prince (from a warboss). Obviously a shitload of green stuff will be involved. I definitely have more to say about the codex itself, I'll try to post some thoughts up in the near future.