In part, this is prompted by the urge to help the 'From the Warp' group get a set of articles on everything. The other part is that I'm amused a lot of people consider daemons 'uber.' In general, though, my aim is to acquaint people with the ins and outs of the Daemon codex, and its problems and possible advantages.
The Daemons have some unique properties in their deployment, a love of invulnerable saves, a distinct bent towards melee combat, and problems in dealing with vehicles.
Most armies set units on the table, and maybe keep a few in reserve for deep strike or outflanking (read: some folks bring Terminators or Summoned Daemons or some such, and some people get cute with Al'Rahem or Valkyries).
Daemons? Daemons are different. Daemonic deployment runs in a few steps...
1) Split the army into two sections
The daemon player takes his army and splits it into two sections that are roughly equal in terms of number of units. IE: a guy brings 2 HQs, 3 elites, 3 troops, and 2 heavy support. That's a total of 10 units, so they may bring it into two sections of 5 units each.
2) Dice off for the 'chosen' section.
Nominate one of the sections. On a 3+, the daemon gets the chosen half. On a 1-2, the daemon get the section they don't want.
3) Deep strike the chosen section
On the Daemon Player's first turn, they deploy the results of the 'chosen' section via deep strike.
4) Reserve in the remainder
On turn two and later, the non-chosen section of the army arrives via deep strike based on reserve rolls.
What does this mean to you?
Daemons deploy in a first-wave punch, and then reserve in based on normal reserve rolls. They're in your face when they want to be.
The Deep Strike, and Icons
All daemon units arrive via deep strike. This means that daemon units are coming in on target 33% of the time, and scattering the rest of the time. There is no good way to predict the direction of the deep strike, and the average result of 2d6 is 7".
Daemons cut down on this by sticking Icons in their units. The Icon permits them to deep strike within 6" of it without scatter.
Generally speaking, the best place to drop an Icon is in a Plaguebearer unit, as they are T5, recieve a 5+ invulnerable, and benefit from Feel No Pain. Any other troop unit is looking at a T3-4 person with a 5+ invulnerable save, so there's a decent chance of just torrenting it out of a given infantry unit.
All units in the codex benefit from an invulnerable save. Generally it's a 5+, though units of Tzeentc and Greater Daemons benefit from a 4+. The Tzeentchian greater daemon benefits from a 3+.
What are the ramifications of this? First off, deep strikes involve the chance of difficult terrain checks. The big red book does not allow you to take armor or cover saves, but lets you take invulnerable saves. Given that most of them have a 5+? The cover is important.
Their Greater Daemons don't really need cover unless psycannons abound; then the Bloodthirster is unfazed and everyone else cries.
Some khorne units can take a 3+ armor save (namely the Bloodthirster and Bloodcrushers) as can the Daemon Prince.
A slight bent towards melee...
The bulk of the daemon army's damage comes from assault. There are only a few shooty units, and only two anti-tank shots in the daemon army. Most any Tzeentch unit can take a Bolt of Tzeentch (S8, AP1) and the Soul Grinder can take an S10, AP1 shot. (The grinder can also take an S8, AP3 blast that acts like a battle cannon, but importantly does NOT get the 2d6 odnance bonus).
Past that? Pretty much everyone in the army is geared towards beating your face in, one way or another. The gods just have different ways of doing it, is all...and here's a short list of them, for sake of an overview. If you want something more in depth, take a look at Stelek's in-depth review.
Khorne favors WS5, I4, Power Weapons (except for the Flesh hounds, but they're actually fast), and Furious Charge. Khorne does NOT, however, know how to equip anyone with grenades. Khorne also has no idea how to be subtle; he just beats you down with power weapon attacks.
Nurgle is the durability god. High toughness on anything other than Nurglings, and lots of wounds on the nurglings. Additionally, pretty much all Nurgle units are Slow and Purposeful and have Feel No Pain. They also tend to pack poison weapons that wound on 4+, and the bigger ones (mostly the Great Unclean One and the Nurgle-kitted Daemon Prince) can pick up a 'wound on 2+' upgrade. Nurgle ALSO believes in offensive/defensive grenades, but that mostly lets them go ahead of powerfists and not CARE when they get assaulted.
Slaanesh is all about speed killing. Most anything Slaanesh has Fleet, rending, and either hit 'n' run or offensive/defensive grenades to go with high intitiative. What's this mean? Slaanesh needs to crank out a high volume of attacks to get the rend in, as usually they're stuck relying on Rending.
The Glaring Problems of the Daemon Codex, and What To Do About Them
The Daemon codex has a couple of major issues that set it at a disadvantage, and there's not necessarily a lot to be done about it.
Daemons and Vehicles
Daemons have...very little...in the way of shooting vehicles to death. S8 AP1 isn't bad, but the shootiest of Tzeentch armies will have something like 3-4 Bolts at BS3, and maybe four at BS4 assuming they went heavy on the heralds. Any other army? They might take an amazing 3-4 BS4 bolts on Tzeentch Heralds, but it's not enough.
That leaves Daemons trying to beat up vehicles. In general, they're looking at AV10. Now, some daemon units can readily crank out penetrating hits. This list includes Greater Daemons and the Soul Grinder, and these guys can even hack up Land Raiders with some luck.
As for the rest of the army...you are trying to attrition vehicles to death.
Assuming that the AV10-rear vehicle is stationary...
The average Bloodletter should land 0.5 penetrating hits on the charge
Plague Beareres? 0.33
Note these numbers are against vehicles with AV10 rear that has not moved. Supposing they move, halve each of those numbers. Against vehicles that moved full speed? Divide that by 6.
Now, the average vehicle taking non-AP1 hits has a 33% chance of dying on each penetrating hit. How many pens does it take to kill a vehicle?
1: 33% chance
2: 55% chance
So, we also have to catch the target. Something like a fast vehicle will never be charged unless it wants to be.
Dealing with this...the Fiend of Slaanesh.
While the numbers suggest that, outside of the Grinder and Monstrous Creatures, the Bloodcrusher is the most adept at killing vehicles, there's something else to consider. You do NOT want to see the numbers against an AV12 dreadnought. Bloodcrushers? Glance it on 6's on the charge; and then they're on S5.
Fiends, on the other hand, can rend dreadnoughts down. Fiends can ALSO catch it. They move 6", fleet d6, and charge 12" as they are beasts. Fiends can also take down Dreadnoughts, which is more than one can say for anything short of a Greater Daemon (as a Dreadnought strikes at I4, which is before the Soul Grinder).
Bottom line of this lesson? If you are playing daemons, TAKE FIENDS. If you are dealing with daemons? Beware the fiends. They are his big way of taking down your vehicles, dreadnoughts, and they can even glance Land Raiders (S5 + 6 +d3 can be 14) with their rending.
If you're going to load up on fiends, well, good idea. All there is to it. I was not initially a fan of Flesh Hounds of Khorne, but against vehicles they're a pretty tolerable run-in-tandem unit. They can bring a similar volume of S5 (on the charge only) attacks to bear quickly, as they have the same charge range as the Fiend (6" move + d6" run + 12" charge). They lack the rending, though, so they can't really hunt Dreadnoughts or Land Raiders, and the few vehicles out there with an AV11 rear give them fits. You can get Rending on the lead hound, which is honestly worth getting only if you're spamming 3 units of hounds, and one of those units ought to be led by Karanak as well. Against vehicles, I'll trade out the power weapons for speed; if I crack them early I get to the meaty bits inside.
If the other guy isn't bringing fiends? Your transports should have a way of attacking without getting the soldiers out, and if your troops aren't out, he's not assaulting and maiming your troops, and that's what Daemons do best.
Daemon Problem #2: Randomness
This is what really gives the daemon codex its problem.
There is a 33% chance they will NOT get the wave they want. Daemon deployment means that most folks will plan a first wave and a second wave, or go with a diluted mostly-symmetrical army (that, incidentially, is not encouraged by the Force Org chart..we get an odd number of elites and heavy support).
The second problem with randomness? Daemons must also deep strike. In essence, you pick a point and 33% of the time the daemons land on it. 66% of the time, they'll land within 2-12 inches of the target area. This becomes more of a problem in two ways: rolling on the mishap table (always bad, but an outright 33% chance of death and 33% chance of useless placement isn't that great) and landing out of position. Since daemons are assault-oriented for the most part, daemons need to land close and get that assault off. (NOte again that Fiends have the speed to get around this a bit...factor in the run move, and in two turns their charge radius is 18 + 2d6).
Now, daemons will take an icon to try to counter this deep strike randomness, but the icon may or may not land where you need it, may or may not survive, and may or may not have a sacrificial unit moved nearby to cloud the 'ideal' deep-strike spot.
What can you do about the randomness?
Not much, to be honest.
Countering Daemon Deployment
Now, this is a potential weakness of the daemon army, and you can exploit it. Daemons MUST deep strike in on their first turn.
Now, the first way you counter this is by winning the dice-off for first turn. Make the daemon player go first. It's a basic tactic, but if you go first, you get to sit around on turn one.
Second, it's a bit more specific to armies. If you have infiltrators or something else, you can try to dictate their deployment...keep them away from you, basically. If you are mechanized, give yourself some room to move. Don't back yourself into a corner; the daemons are fine if you lose your mobility and they can hem you in.
Daemon Primer Conclusion
Deamons are a nasty army that play significantly different from other armies. They deep strike in as one, then trickle in. They have a lot of outlandish units, and a lot of ways to surprise you if you're not ready for it. However, they have problems in dealing with vehicles, and their very deployment can be a pain (...there is nothign quite like deploying a Soul Grinder, scattering it into difficult terrain, and then rolling a '1' and watching it not move for a game).
I do play some daemons, and they're more of a fun army. They can win, but it's an uphill battle against a 5th-edition-conscious list and player.