Night Goblins were an excellent unit in the old army book and remains one in the new incarnation as well. For a very cheap cost, you get a mob of sneaky gobbos with some of the best unit upgrades in the Warhammer world.
When running Night Goblins, the first decision to make is whether to bring a mob for combat or a mob for supporting, as this will greatly influence how many gobbos to get for the mob and what sort of weapons they will be carrying. It is my opinion that Night Goblins should be bought either in large mobs that will outrank the enemy, or in the minimum amount (20 gobbos) to keep costs down.
If you do decide you want a mob of goblins fit for fighting, the second decision is whether you hope for them to defeat a not-too-nasty enemy on their own (with some assistance) or if they are meant to act as a steadfast unit that keeps nasty and expensive enemies in place while you smash the rest of his stuff, before turning to help the gobbos win the day. Both strategies have merit and are quite do-able, but it is quite important to equip them properly and deploy them accordingly for what you want them to do.
Night Goblins are absolutely wonderful at dying in heaps without running away, as long as the Warboss is within barking distance. This way of using gobbos is probably the most common one and you'll encounter them in most competitive lists. The idea is to deploy your mob in ranks 5 wide, 8 to 10 ranks deep, and get them into combat with nasty units of far more expensive enemy models, who although they should defeat your gobbos in combat, won't have even near as many models in their unit to prevent you from being steadfast. Such goblins units are best given shields and handweapons, since they get a parry save no matter the strength of the opponent and still want to prevent casualties. Spears are probably not worth it on such units, even if they are free, since we're only talking an additional 5 WS2 attacks, and the gobbos are there to keep enemies in place, not to actually win combat. Nets can be useful for such units, but considering that you get 15 more gobbos for the same price as equipping them with nets, you might be better off keeping the unit cheap.
A Night Goblin unit that hopes to lay the beat down on enemies should be prepared to make use of the Horde rule, meaning they'll go 10 models wide and benefit from an extra rank of attacks. They will need at least three ranks, preferably even a spare rank for taking casualties without losing attacks. These guys actually hope to defeat their enemies in combat, strange as though it may sound (being goblins and all) and can do reasonably well with only a little assistance. The mob should definitely consider switching their hand weapons for spears, as the wide facing of your mob should mean at least 7-10 additional attacks. A timely spell of The Gift of the Spider God, granting your gobbos poisoned attacks, will make for a very nasty unit with all those spear thrusts coming the enemy's way. The mob can also benefit greatly from a few cheap Goblin Big Bosses with great weapons, providing added killing power with several S6 attacks and the extra leadership. If you plan on having bosses lead them, Nets become a very good options despite the hefty price. It makes your goblins effectively as tough as their orc counter parts, and increases the combat proweness of your mob by quite a lot.
The third way of fielding Night Goblins is bring them as support units, cheap throw away units that give you additional deployments and are then mostly used to deliver fanatics, act as road bumps and redirecters, or possibly hunt down war machines in the later stages of the game. Such units are best fielded as cheap as possible, meaning only in units of 20 and given as few upgrades as possible. It can be a good idea to give these gobbos shortbows and deploy them in two ranks early on. They won't kill much, but 20 arrows can still take out swarms, fast cavalry and other annoying support units in the early turns. If you have the points to spare, it can definitely be worth giving them a Musician for swift reforms, the better to get said enemy units within line of sight. Since they're such cheap units, you don't have to think twice about sacrificing them if it serves your schemes, and gladly use them to tie up enemy units or cavalry for a few rounds, since even though the gobbos will die in heaps they should still be steadfast for a while.
Unit size: If fielded as a Steadfast unit, mobs of 40 are good, deployed 5x8, and will keep enemy elite units in place for a long while. If instead brought as a fighting Horde, mobs of 50 are definitely worth getting, especially if given spears. For support units bring only the minimum of 20, as bumping them up to 25 or 30 will only cost you more without giving you any obvious benefits.
Equipment: Steadfast units only need their shields to hide behind, hoping to frustrate the enemy by rolling plenty of 6's for their parry saves. Nets can be worth it as it will keep them alive for longer, but can skipped if spare points are hard to come buy. Horde units should definitely consider getting both shields, spears and nets to aid in breaking the foe. Support units are ideal for carrying shortbows, but can do alright with shields as well.
Command Options: Steadfast units can make good use of both Musicians and Standard Bearers, as they still want to fight and both upgrades help a lot with that. The same goes for Horde units, but since the Boss upgrade checks in at the price of 3 extra gobbos you might only want to bring one if your unit includes Shamans or Big Bosses that prefers not to be challenged. Tiny support units will die to almost anything so giving them Standard Bearers is usually a bad idea, as is putting characters in the unit so bosses won't be necessary either. Musicians can be well worth taking for added mobility and to help rally from panic.
Fanatics: A lot can be written about Fanatics and gathered up into a stand alone article, which I intend to do eventually. Worth mentioning though is that the new rules only require enemies to move into fanatics in order to suffer the extra D6 hits as the gobbo dies, meaning it became a lot easier to catch charging units with your fanatics. A rule of thumb when buying fanatics for your Night Goblins is to never spend more points on fanatics than you've done on the unit itself, unless it's part of a very, very sneaky plan. There are few enemies out there that 3 fanatics will kill that 2 fanatics won't cripple, so make sure to spread your fanatics out across several mobs if possible. It makes for a less predictable army, and will usually require more enemy sacrificies in order to draw them all out.