Chaos Spawns are weird and wonderful creatures. Model wise G.W just give you a body and a crap load of bits and say "here, knock yourself out". In game they appear average, time to take a deeper look.
Know your slime bag
Chaos Spawn is a reasonably tough and strong model (S5 T5) with three wounds a piece. Not the fastest initiative wise or the best weapon skill. They are leadership 10, though they are fearless so this doesn't matter a huge amount. The negative for Chaos Spawn is they have no form of armour save and have D6 attacks, so you could roll a 1 or be shit hot and roll a 6.
Chaos Spawn also cause fear and have the rage special rule, so they will get +2 attacks when charging (very nice if you roll a 6 for random attacks) and if a unit fails fear test then those Chaos Spawn will be hitting on a 3+.
Now the lack of cover save is a bit worrying, but Chaos Spawn are beasts which means they can move through terrain without any hiccups getting at least some form of save. Still, it is an issue in close combat.
Mutated puss bags
Chaos Spawn have a cool rule which comes into the effect at the start of assault. You roll a D3 and could get 4+ armour save, 2D6 attacks pick the highest or poison. All are pretty reasonable and nothing to grumble about.
Upgrade yo' drool machine
Chaos Spawn can be given a mark of a chaos god, some are good and some are pretty pointless.
Mark of Khorne - this gives rage and counter-attack. It is pretty cheap, but Spawn already have rage so you're paying 2 points per model to get counter-attack.
Mark of Tzeentch - gives a 6+ save. Considering what you get I would leave this at home.
Mark of Slaanesh - +1 initiative, which makes the Chaos Spawn I4. I would probably leave this as Chaos Spawn has no form of assault grenades and only I3 so doesn't matter if striking last through terrain anyway.
Mark of Nurgle - +1 toughness, so makes the Chaos Spawn T6. This is the best upgrade as Chaos Spawn are almost immune to instant death and only anti tank weapons stand a reasonable chance of wounding themm. Small arms fire and basic close combat attacks will more than likely bounce off them.
Run, Slob Face, run!
Chaos Spawn are best in units of 4-5, 5 if you can afford the points and as mentioned above the mark of nurgle is the best mark out the load and really increases their survivability.
Spawn need to hug terrain like their new best friend due to lack of armour save. Use it to block LOS or get some form of cover save. Should be pretty easy as only need 25% covered. Remember in area terrain models do not need to be 25% covered, so Spawn could race up moving from area terrain to area terrain.
Chaos Spawn are best run in pairs if possible, which I know is tough because Heldrakes conquer the fast attack section. They work well with Rhinos, foot sloggers and other fast moving units such as Maulerfiends. While the rest of the force advances behind the Chaos Spawn can move up and act as a cheap distraction taking off pressure from other elements of your army.
Primary targets for the Chaos Spawn would be small camping units such as Devastators, Grots, Scouts, Combat Squads etc. All these units stand very little chance against a unit of 5 Chaos Spawn lumbering in their face. With Chaos Spawn tackling these units you do not need to worry about missiles coming down your face as those units have been tied up or destroyed or need to be concerned about an objective in the opponent's back field - that's not your opponent's problem, as they thought the had it secured.
As Chaos Spawn are such a fast moving unit they are great for linebreaker too, especially if going after targets mentioned above.
Another valid tactic for Chaos Spawn is monster hunting and tarpit. As Chaos Spawn don't have an AP in close combat they could tarpit something like a Riptide and it will take the Riptide forever to get rid of them as the Chaos Spawn cannot be instagibbed by smash attacks as they are T6 ;)
If infantry isn't available then Chaos Spawn can easily bash vehicles in; they are S5 and most vehicles are av10 on the rear. Remember to block the exits on a vehicle, even against a Rhino this should be possible as Chaos Spawn are on 40mm bases and should be able to cover the doors half ringed around the back of the Rhino. If you do this then the enemy unit has to emergency disembark, so no shooting or moving or assaulting next turn - nothing! If the tank blows up then limited models can be placed as cannot be within 1" of the Chaos Spawn.
If none of the targets above are available then Chaos Spawn are great for mashing up your bog standard troops; the high toughness should keep Chaos Spawn alive while their S5 and reasonable amount of attacks should tackle troop units. You can use this method for them to move up the field and move to the primary targets as mentioned above.
For your information:
5 x Chaos Spawn vs 10 x Tactical Marines -
Chaos Spawn roll a 4 for attacks on 2D6, they are charging so get 6 attacks each (rage) that's 30 attacks.
Tacticals attack with 12 attacks - hit 7.92 - wound 1.26 - that's a wound off a Chaos Spawn.
Chaos Spawn attack back with 30 attacks - hit with 15 - wound 9.9 - 3.36 dead Space Marines.
Overall that's not too bad for the Chaos Spawn. Imagine if the Chaos Spawn got poisoned attacks or even rolled a 6 for number of attacks!
Mutant bros for life
Sometimes Chaos Spawn need a buddy and there's one guy who I think will get along with them nicely; a Nurgle Lord. Give your Nurgle Lord a bike, power fist and burning brand and he can speed up with the Spawn and burn some infantry or split off and do his own thing. As the Chaos Lord has a power fist he would be better for smacking vehicles or small units of infantry so doesn't become stuck in a tarpit.
Chaos Spawn, do you think they are the cool mutants or sad evil ones?