Bloodletters of Khorne

In a nutshell: Power weapons on basic troops! The 40k universe turns upside down, news at 11!
Well there's gotta be some kind of downside, right? Read on!

Pros: For a little less than a Vanilla Space Marine, you get WS5, 3 attacks on the charge, and power weapons. Seems like a good deal to most people. Let's not forget they have furious charge, so they go first...hit on 3+...wound on 3+....yeah, they are a real beat down for marines.

Cons: Mobility. 5+ save. If regular marines can be torrented to death and they have a 3+ save, what do you think will happen to guys with power weapon attacks? You guessed right, death en mass from everything the other guy has. Boo!

Tactics: Run at enemy, hope you make it. Sadly, while quite dangerous to basic troops (and even basic AV10 vehicles when charging) they aren't exactly hard to get rid of. So, how do you make these guys work for you instead of being annihilated? Why, multiplicity of target syndrome, of course. Works best in a Khorne army, where you might be slow you can offer a lot of power weapon attacks on every unit you've got. ;)

Final word: In a pure Khorne army or a (gasp, umm...what's a heretic to a heretic? umm..believer!) mixed Khorne/Tzeentch army you can make use of these guys. Sadly if you try to use them as anything but a minor threat, you'll find yourself back in the warp before you can scream 'Blood for the...wtf!'. ;)

Daemonettes of Slaanesh

In a nutshell: One of the troops available to a Daemon army, sadly they really aren't any damn good. Think about it, if they had power weapons...would you take them? Yeah, Bloodletters are still better. It's like someone designed this unit during 4th edition years ago, and never revisited it when 5th revised rending.

Pros: High Init, 4 attacks on the charge, fleet, super nerf grenades. They are moderate threats against light infantry like Guard, and can drag down Marines through sheer number of attacks.
The short list ends right about there.

Cons: They can't really handle any kind of CC threat or rapid fire pointed their way, T3 just doesn't cut it. They can't kill even basic AV10 tanks with any reliability. They can be safely ignored by most armies, I've moved tactical squads right next to them and shot a more threatening unit to pieces and taken the charge.

Tactics: Since they aren't point costed appropriately at a more sedate 10 points, your tactics are rather limited. You can bring a bunch of them, and try to swarm the enemy with your 'horde' of Daemonettes. You'll find yourself in for a rude surprise when 60 marines quote unquote 'swarm' you back. Or 180 Orks/Gaunts. Your best tactic is to not bring this underperformer in anything but a themed army.

Final word: Great model, the one previous. The others are all just...either ugly or some kind of pathetic attempt by GW to cover up their shame. It has inadequate rules and is too highly priced to be of much good to any Daemon player. Skip this turkey.

Nurglings

In a nutshell: What was once annoying as hell in 4th edition, is just very irritating in 5th edition. They are very hard to get rid of in terrain when shooting at them, and make for excellent objective contesting units.

Pros: 3 wounds, 5+ invulnerable, immune to instant death, can use swarms and go to ground to take themselves to a 2+ save...talk about serious pain in the ass.

Cons: Utterly useless in doing just about anything else. Can't really CC anyone to death, but they can tie people up for a very long time if there's a lot of them. They can however get smushed by a solid CC unit in no time flat.

Tactics: Use them to contest objectives in midfield, and soak up fire in nearly unlimited amounts when sitting in terrain and going to ground. Just don't forget to spread out, you are vulnerable to blasts after all--Tau will ruin your day if you forget. ;)

Final word: You can if you wish really dump points into your non-troops choices by taking 2 min size Nurgling squads but it does make things quite risky in winning missions. You should really use Nurgling squads as a cheap way to boost your total army size. It does increase your total KP count, but they are difficult to earn so I think it's worth taking them in almost any Chaos Undivided army.

Pink Horrors of Tzeentch

In a nutshell: Semi-shooty anti-infantry, in a world where everyone has 4+ cover saves and in an army with plenty of power weapon armed units. Definitely has a niche purpose if you want a shooty army, but if you really want to kill the other guy for sure--the power weapon squads seem like a better choice.

Pros: 3 BS3 S4 shots @ 18". Can move or fire, so for shooting at infantry it is quite fine. As a backup, you can (and should usually) give them a anti-tank shot. Does make the rest of the units shooting useless, but gives you more flexibility to handle transports from 24" then followup against the infantry inside the next turn. Remember you need multiple units to do any reasonable damage to vehicles due to the low strength and low accuracy. They also have a 4+ invulnerable save, which is good when out in the open.

Cons: T3. T3. T3. People, when you land this unit on the table and the other guy has so much as a IG grenade launcher--do not fire! RUN! Spread that unit out. You can be annihilated by blasts if you really want to, but you probably don't want to just give up a unit for nothing. lol

Tactics: Drop and roll (run) is my advice. Much as people want to be able to drop down and roll a bunch of dice, being less vulnerable to blast, ordnance, and flamer templates is much more important to having said unit after you pick off 3 Orks, ok? When facing a mech army, sometimes you have to suck it up and fire those bolts at max range...which is better than someone moving a transport full of rapid fire troops on top of you and unloading. You should pretty much always stay at long range and spread out as much as possible, firing at enemy infantry. You cannot fight in CC, ever. Everybody kills you. lol

Final word: A moderately useful shooting unit, not very strong in attack or defense. The 4+ invulnerable save is neat, of course you could just get a 4+ cover save in terrain. Call it the middle of the road shooty unit, able to shoot a bit but unable to CC at all.

Additional information:

I almost forgot the Changeling. If you bring a Pink Horror of Tzeentch unit, you probably want to consider him. Especially in tournament play. He can make your opponent fail LD tests and be unable to shoot. Of questionable use against most armies due to their high LD (like all the other LD bending abilities as most negative modifiers have been removed from 40K5E) but it can make your initial drop a little bit smoother against a heavy gunline army. In friendly play, you're probably already earning dick points for bringing a turn 2 auto-assault power weapon horde against your friends...so don't be a supercock and bring the Changeling too, so they can't shoot you either. ;)

Plaguebearers of Nurgle

In a nutshell: Best troop available to Chaos Daemons. For 5th edition, they are very difficult to get rid of and for troops in objective missions that is awesome.

Pros: T5, Feel No Pain, 5+ invulnerable...totally can't kill you, but unless you are facing a crazy shooting unit (guided/doomed scatter laser war walkers) or a good CC unit (terminators) they just won't drop. Stick them in cover for extra fun, go to ground...and you have a 3+/4+ save. Talk about annoying. It's the Nurgle theme song: You can't kill me, nyah nyah. lol

Cons: Well, they suck in CC but do at least scare Wraithlords and C'Tan a bit, since they can wound them on a 4+. They cannot shoot either. Hard to kill is pretty much all they've got going for them, don't ever forget it.

Tactics: Drop, run into cover. Remember you cannot FNP deep strike casualties, so it's probably best not to drop into cover. Nurglings can suck it up, these can't. They can tie people up in CC, but they need support from other elements or they'll be whittled down slowly but surely. So either hide them, or hide behind them and counterattack.

Final word: Most Chaos Daemon armies should have Plaguebearers. All the other SCORING troops are easily eliminated, and in 5th edition...that's one way of winning and without plaguebearers, no matter how mighty the rest of your army might be--it's one you will be vulnerable to. Use them to soak up fire from the rest of your army when scoring troops are on the line, but make sure you protect them from annoying things like Thunder Shield Terminators, who like to eat Plaguebearers for breakfast. ;)