Nurglings; teeny tiny bits of grossness that are much beloved by the Greater Daemons that spawn them. Also the only unit in the entire Codex that has the Infiltrate special rule (more on that later). I know I've taken to discussing profiles and costs towards the end of a post but there really isn't much to discuss in terms of equipment and upgrades for these little rotting imps because they have no upgrades apart from being able to add extra bases to the unit. For 45 points you get three swarms of Nurglings which for what you get isn't too bad and to add an additional swarm costs 15 points (you can add 6 additional bases for a total of 9 per unit). Now you might be wondering why you'd want Nurglings considering they have no upgrades, you don't get as many bases to the unit as you do with the other four troop choices and, if I'm totally honest, they're a bit of a bitch to paint up. The reason is that they have pretty decent stats for a troops choice. What we're talking about here is a WS, BS, S, T and I value of 3 each, Ld 7 (pretty standard for the Troops choices) but what really sets them apart is the fact that they have 4 attacks and 4 wounds PER BASE. So for a basic unit of three swarms you've got a cheap unit with 12 attacks and wounds and for each base you just add on another 4 to each so when there's 9 swarms in one unit that's 36 attacks and wounds from a single unit for 180 points. This makes them up to be the same cost as a maxed out but not upgraded unit of either Horrors, Plaguebearers or Daemonettes and cheaper than Bloodletters but with more attacks (apart from the Daemonettes) and wounds. Now the trade off here is that they're not as strong or tough as the other troop choices but I guess if they were then they'd be ridiculously overpowered for what they are.
As baby Daemons of Nurgle they have the appropriate special rule which, as I outlined in my Plaguebearers post, gives them the Shrouded, Slow and Purposeful special rules and gives them Defensive Grenades. I won't go into detail again on them because I did it before and frankly I'm feeling too lazy to even copy and paste but what I will do is go into their other two special rules; Infiltrate and Swarm. The Swarm rule is a bit of a double edged sword because it allows for them to treat Difficult Terrain as normal Terrain (I guess because if they were real they could just move through/over it without any trouble because of them being tiny) but for that benefit there's a price which is that template (including Blast/Large Blast) weapons that cause an unsaved wound cause 2 wounds instead.
The Infiltrators special rule is an interesting choice for this unit. How it works is by allowing the unit to deploy after all other units have deployed (the rule book says that if both sides have infiltrating units then roll to see who deploys theirs first) and they can be placed anywhere on the board as long as they're not within 12" of an enemy unit and not in line of sight. Alternatively they can be placed anywhere at all, even in line of sight if they are 18" away from an enemy. Also the rule states that any unit with this rule (and I'm assuming only those that use it) cannot charge on the first turn. GW have got a lot of that going around in this turn and I guess that it is to stop armies such as the Daemons from wiping everyone out in two turns.
This post is as short of a Nurgling itself because there aren't any options to discuss. The way I see this unit is that they aren't designed to cause a lot of damage but are designed to tar pit powerful units; perhaps the unit with the heaviest fire power. In this way they act as a shield by absorbing hits and locking heavy hitting units in combat while the rest of the army is advancing without the fear of getting blasted apart before getting themselves into combat. I'm also guessing that this is the reason that Nurglings can't take an Instrument or an Icon because otherwise you'd effectively be allowed to begin deploying an army anywhere on the board which wouldn't be fair. The great thing with Nurglings is that the unit size doesn't really matter as they can perform the job just as effectively due to the amount of wounds. These are also a good choice if you want Daemon allies and you're after a specific unit as they are a cheap option to fill the mandatory troop choice without spending too many points. They also seem to be scoring units so they can fulfil the same role as a Plaguebearers as objective holders and will cost 90 points to get double the amount of wounds of a full contingent of Plaguebearers (although you do lose Poisoned weapons). I'd say use these as a tar pit to hold up either the most powerful ranged unit they can get to, spread out and block line of sight or take on some characters and hope for the best. Either way they should assist in the survival of the rest of the army while they get themselves into position to dish out their particular brand of hurt.
EDIT: As you can see below it has been pointed out to me that Nurglings are NOT scoring units so can't hold objectives as well as Plaguebearers but they can be used as denial units which will stop an enemy from scoring with an objective which can be useful in certain circumstances. Thanks for pointing it out to me.