First of all, lets look at their stat line. They've got a standard Space Marine stat line which is pretty decent, but they get no ATSKNF, Combat Tactics, or Chapter Tactics, and are only 2 points less than a standard marine. The thing we do get over our loyalist brethren is that we have lots of options.
So first for our options are the marks of chaos.
- Mark of Khorne - This mark gives you the Rage special rule, so should really only be taken if you're going to be outfitting them for combat. My favorite way to run Khorne Marines is to take a squad of 9 or 10 (depending on if you're bringing a character to go with them) and put them in a Landraider. Swap out their Bolters for CCW for free, give a couple guys Meltaguns and a CCW and of course Meltabombs on your Champion, and maybe throw in an Icon of Wrath in there as well. If you feel like you may be fighting Space Marines you can throw in Veterans of the Long War as well. What you end up with is a unit that gets 4 attacks per guy on the charge at strength 5 with re-rollable charge distances and Hatred (Space Marines) for just over 200 points, plus the cost of your Landraider. A bit pricey, but they will mangle any non-dedicated close combat unit, and can hold their own against tougher things as well. The Landraider will also help them actually make it to the fight.
- Mark of Slaanesh - This mark grants your units +1 Initiative. Now you can do something similar as with the Mark of Khorne, but with this mark, I prefer to use it to open up the option for an Icon of Excess. The Icon is a whopping 30 points, but gives you Feel no Pain, which can make for some tough objective campers. I like to bring either 10 in a Rhino with the Icon, or 15 if you're just going to sit on a back field objective. Of course, with the extra Initiative, you may want to send them into combat, and the Icon is a big help there as well. Mark of Slaanesh is really excellent as it is so versatile.
- Mark of Nurgle - This mark will give your troops +1 Toughness, which makes them ideal for taking or holding objectives. Generally if I want tough troops, I'll just go with Plague Marines, but for the Price of a Landraider you can have 10 of these bad boys in a Rhino with some Plasma Guns to give them some firepower. Not a bad option, but I generally wouldn't take it.
- Mark of Tzeentch - This is really kind of a loser mark. It gives you a 6+ invulnerable save. I don't really see a time when this would be very useful, so I can't give any advice on taking it. A Tzeentch themed army is the only reason I would take it. Seriously, don't bother.
- Icon of Vengeance - This Icon gives you the Fearless special rule. It would almost be good, if it didn't cost so much.as it is, you could get 2 CSMs for the same price, and you wouldn't also lose your ability to go to ground. For 10-15 points it would be good, but as it is, I wouldn't bother.
- Icon of Wrath - This is for units with the Mark of Khorne only and gives them Furious Charge and allows them to re-roll their charge distance. This Icon is one of the better ones in my opinion, at least for your standard marines. It's a little pricey, but not at all bad for what you get. I would take it if I were running Khorne Marines.
- Icon of Excess - This is for units with the Mark of Slaanesh. It gives them Feel no Pain, which of course is awesome! It is the most expensive Icon, at 30 points, but it really can dramatically increase your survivability. Definitely an attractive option
- Icon of Despair - Mark of Nurgle only, this one is garbage. 10 points nets you the Fear special rule for the entire unit. That's it. Fear is such a useless rule, I would never take this Icon.
- Icon of Flame - Tzeentch exclusive, this Icon grants the unit Soul Blaze for any Bolters, Bolt Pistols, or Combi-Bolters in the unit. I've never actually seen Soul Blaze do much, so as far as I'm concerned, this one is just a waste of points.
Next I'll talk about Veterans of the Long War. Not a terrible upgrade for only 1 point per guy, but only really useful against Marines. It gives you +1 Ld and Hatred Space Marines, including all their non-codex buddies like Space Wolves or Dark Angels. Really, it depends on if you think you'll be fighting Marines in close combat.
Now for weapon options. Any Marine can swap their Bolter for a Close Combat Weapon for free, or you can take a Close Combat Weapon for 2 points. I generally would only swap out, as taking both tends to be a bit unnecessary. you also can take a Flamer, Meltagun, or Plasma Gun, and if you have 10 or more guys you can have another of each of those or choose fromaa Heavy Bolter, Autocannon, Lascannon, or Missile Launcher with Frag and Krak Missiles. I'm not really a fan of taking a heavy weapon as it really limits your mobility. I prefer to take either 2 plasmas or 2 meltas. If we could combat squad, I think you would see a lot more Chaos Marines toting Lascannons and stuff, but as it is, it doesn't seem to be worth it.
Honestly, if you're looking for a cheap place holder unit to fill up some troop slots, Cultists are probably the way to go, but outfitted correctly, Chaos Space Marines can pack a punch, and they tend to be better for taking objectives in the mid-field or farther up the board. They also can be taken in big mobs, so if you have lots of these guys kicking around, try playing around with big 20 man squads. They can sometimes be a pain to shift, particularly with a character in there to keep them from running away. If you haven't taken these guys in awhile, try giving them a shot. Sometimes they can do pretty well, especially if the circumstances are right.