or, how I learned to stop worrying and start worshipping Slaanesh
Aside from Orks, the faction I've played the longest in 40k is Chaos. It started back with the 2rd edition Chaos Codex, a massive beast of a book; If you could not best your opponent on the tabletop, a blow to the head from this book would surely grant victory. Perhaps my memory has embiggened it just a touch, but it certainly captured my imagination. My chaos army was a renegade chapter named the Warp Falcons, lovely purple fellows, and they had units aligned to each of the chaos powers. I seem to recall my stars being the plague marines, who were, at the time, single pose plastics. Good times.
3rd edition came with new miniatures for each of the 4 chaos powers, and the noise marines drew me in. After all, what's cooler than sonic weaponry? Answer: nothing. My Emperor's Children were basically an entirely new army, and as I was purchasing them I made sure to order plenty of extra sonic weapons. I still have some in a bag today, somewhere. These guys were horribly painted, but I loved them, and they served me well on the battlefield.
When the 4th edition codex arrived (well, late 4th, proto 5th), I reorganized them accordingly. The ability to upgrade most of your army to carry sonic weaponry was sadly gone, however so were the restrictions on mixing your gods. The Emperor's Children took this opportunity to strike a deal with a sorcerer of the Thousand Sons, and a few of the soulless warriors joined their ranks. My first trip to the doubles tournament provided the inspiration needed to repaint them. This is, quite predictably, a job still ongoing.
Recently, after a good few months of playing Iron Hands and Orks, I've gotten the urge for Chaos again. I took them out on the battlefield a few days ago, where they squared off against my brother's Salamanders. A draw, but considering my horrific reserve rolls left me with a good portion of my army entirely absent, clearly a moral victory! (what do you mean I always claim moral victories?) Anyway, the army is pretty good at what it does, nasty up close but with enough long range fire power that I don't need to struggle across the table to be effective. The chaos book is still very solid, even if I am ignoring a bunch of units because they don't really fit my theme (berzerkers, plague marines, defilers...actually that last one is more because I'm not a fan of the model). However, with the Blood Angel codex imminent, I've been doing a lot of thinking about switching over to the new list. Should I? Shouldn't I? If I do, I can play with things like the new drop ship, and dreads that wouldn't try and kill me, and land raiders that can hold more than 10 dudes. But I'd also lose stuff like my kickass lord, and sonic weapons, and my thousand sons squad. I've been flipping back and forth on the idea for a while, but there is one thing that draws me to the idea the most; the new BA codex just looks more fun. It might perform still, but the chaos book doesn't pack the tricks and options the newer books give you. I'll wait and try a few proxy games until I make my choice, of course, but the appeal of a fast-moving deep-striking out-flanking force is pretty strong. I've got some good ideas for "counts-as" death company, too.
Having looked briefly through the BA book, here's what I'm thinking of for my proposed core.
Sanguinary Priest with power weapon and jump pack;
joining a 10 man assault squad with 2 flamers and a power fist sgt,
2x 10 man tac squads in rhinos, with meltas and multi meltas, one with a fist and one with a sword
Las cannon Predator
This is, with a couple of exceptions, doable with minimum amounts of new miniatures needing to be scavenged/constructed/converted, and by my estimation comes out at around 1100 points, which should provide a solid core, but still have room to add some new toys. We'll see what happens.