I would advise against all the options except where I specifically state otherwise.
You must choose your troops carefully- after getting the other "If-I-don't-get-these-units-I-can't-be-competitive" units, you won't have a whole lot of points left over for scoring- and most of these troops can do more than score- for example...
Bloodletters of Khorne: Can be very good
Bloodletters are very, very killy, and not horrifically expensive: once they close with the enemy, they pretty much rampage. However, they are very fragile, and about 25-75% die on the first drop: plan for this, and they will work well for you. Don’t rely on them to hold objectives passively: they are, however, good at wiping a unit off an objective and claiming it for themselves, or pulling a unit off an objective- oh, you don’t know how that works? Move 1 guy within assault range of your opponent in the movement phase, then when you assault, and they “defenders react,” they have to move 6 inches to get as many people in B2B as possible: this could get them off the objective. :D devious, I know.
I'll go over more ways to use them with other units after the main review.
Daemonettes of Slaanesh: Very useful if you use them right.
Let’s compare them to bloodletters, because they fill similar roles. For 2 pts cheaper, they have fleet, grenades, rending, 3 attacks, and I6, but are S and T3, and WS4... I don’t know. They can’t be swarmed properly- they’re too expensive- and they die really, really easily. They’re just not as good as bloodletters in my book.
I would say, if getting these at all, mix-and-match with bloodletters. While bloodletters can go toe-to-toe with some of the best close combat units in the game (Against power weapons, they're as durable as terminators, and seriously- what decent close combat unit doesn't have power weapons?), Daemonettes are a bit more versatile. They're fast and have grenades, so they're MUCH better at clearing out that annoying heavy weapons team camping in cover- or, for that matter, charge a rhino and hope for sixes! With 4 attacks each on the charge, you could even kill it!
They crumple against actual close combat units, but they slaughter entrenched, difficult-to-remove units admirably and efficiently.
These are very, very solid troops, and I mean that in the most literal sense of the word: even a 5-man squad isn’t going ANYWHERE. They’re cheap, durable scoring- what more could you want in a troop? I’ve had a shooting-based eldar army fire EVERYTHING at a 5-man squad 7th turn, and 2 lived to claim an objective. Absolutely brilliant. Use them a few times, and you'll see what I mean.
Beginners, give them an icon, they're the best platform for it.
Pink Horrors: Very useful.
I use pink horrors to hold midfield objectives, and plaguebearers to hold backfield ones: pink horrors can contribute to the fight in a non-ignorable way while holding an objective: mowing down infantry that just got shot out of their transports by Chariots of Tzeentch, and other things. Get Bolt: it’s good to get Bolt wherever you can, and this is a pretty cheap way to do it. Suuure, it’s BS3: but that’s usually one more dead rhino it it hits, and being in the midfield, there’ll be no shortage of rhinos for it to shoot at. Each squad usually kills 1-3 AV10-12 vehicles in each game.
Also, pink horrors are surprisingly durable: that 4++ is very nice, and means that they can hold objectives in the open with no fear. Well- as much fear as as guardsmen hiding in the burnt wreck of their chimera. They can take a volley or two, but don't push it.
The Changeling is a great choice: for 5 pts, the ~27% chance to control any space marine squad within 24” is really good.
Nurglings: I really can't comment or Devastating!
I really don’t use nurglings much, because they don’t do a whole lot. They are the cheapest unit in the book, and being an Eternal Warrior swarm is pretty useful for tying up units. See if you like them: use them to hold down units until your Khorne and Slaanesh stuff can get the charge on them. I could never get the hang of it, maybe you can.
NOW, in an epidemius list... get them. In the beginning, they can tie up threatening units that could rampage through your initially vulnerable army, and maybe get a casualty or two for the tally, but as the tally goes up, they get NASTY- with noxious touch, suddenly everything is making a lot more saves, and failing them. With Power Weapons... they are absolutely vile. 13 points for 3 power weapon attacks that wound on 2's? YES.
Course, by then, you're already winning most of the time against armies like GK's or SW, but it can turn the tables against high-model-count armies.
Next is Elites, the make-or-break slot!