Warhammer 40,000/6th Edition Tactics/Necrons

Why Play Necrons

Necron Lord - Your basic HQ choice. His main job is to carry a Resurrection Orb to help your Necrons stand back up. He can take the fearsome Warscythe (and I suggest you do) to make him nasty in hand to hand combat. He also has access to several other nice pieces of wargear, such as the Veil of Darkness, and the ability to take a Destroyer Body to become a T6 Jetbike.

The Following are special characters

The Nightbringer - A hand to hand monster. He's S10, T8, with 5 attacks and a 4++ save. Nasty. He also gets a 24" range Lascannon, and the ability to knock weedy gits aside, or simply make everyone around him explode.
The Deceiver - Almost as nasty as the Nightbringer in Hand to Hand, the Deceiver's power comes from being able to force your opponent to take moral check, simply walking out of combat, and allowing you to redeploy at least one unit before the game starts.

Flayed Ones - Can Infiltrate or deepstrike. Forces foes to take a LD test or hit them on 6s. Are essentially marines with 2xCCW. meh.
Immortals - Bigger, tougher (T5) shooty-er (S5 assault 2 guns) and more expensive Warriors. Most armies will have at least one big unit of these guys.
Pariahs - Fearless immortals. With Warscythes. That knock all nearby enemies to LD 7. And force psykers to take LD tests. Sounds awesome, right? Pity they're not "Necrons" and, as such, don't benefit from all the cool special wargear and rules... They tend to get shot to pieces before making it within 12" of the enemy... Easily the best H2H unit in the Necron book.

Necron Warriors - Your bread and butter. You want a lot so you don't phase-out. Marine-like, but worse in hand-to-hand. Get to stand back up.

Destroyers - Necrons with bigger guns on a jetbike. useful for fast, mobile firepower to threaten flanks or isolated units.
Wraiths - They're fast, have 6S, and a 3+ invulnerable save. But that's it. They'll get either shot (only 1 wound) or stuck in combat where they'll then die. Skip 'em.
Scarab Swarms - Fast as all get out. Turbo-boost for a 2+ cover save. Charge stuff to bog it down indefinitely. Very, very helpful. Also quite cheap.

Heavy Destroyers - A very expensive lascannon on a jetbike. not many people use these...
Monolith - Have at least one for larger games. It's really difficult to kill, helps your guys stand back up, and lets you get your necrons out of dodge.
Tomb Spyder - A T6 monstrous creature. Can generate Scarab Swarms. (though might take a wound) Low WS and number of attacks make them pretty mediocre in hand to hand, but still one of the best offerings the Necrons bring to the melee table.