But, as with just about everything else in this codex, you have to look at the bigger picture with this guy. He has amazing synergy with everything else in the Necron army. Ultimately, however, he boils down to two main roles: Shock Absorber and Damage Control.
Let's take a look at Shock Absorber first:
I've already mentioned the Deceiver's great durability, and made the point that Wraithlords fulfill this same function for the Eldar for less. But here's the thing: when an Eldar player's opponent shoots at a Wraithlord, he's using weapons designed to take down vehicles. However, with upgrades like Holo Fields and Spirit Stones, Eldar vehicles are already very durable... They really don't need a giant pincushion to protect them. However, Necrons don't really have vehicles for those anti-tank weapons to fire at - the only option is the Monolith, but most of those weapons barely affect it, if they even do at all. This means that, in an army without the C'Tan, those lascannons, rockets, meltas, etc, are going to be firing at one of two things: your Destroyers/Heavy Destroyers, who have such low model count that the entire squad can easily be taken down by a few high-strength shots, or your Warriors, preventing your all-important We'll Be Back! roll. Sure, they'll only take a few down per turn, but those ones are going to be staying down for good.
With the Deceiver, however, everybody panics. They find out that he's got 5 wounds and a 4+ invulnerable save, and they start getting crazy, thinking that they simply have to bring that monster down. Often, new players will go nuts over the idea that he ignores invulnerable saves with his 4 S9 attacks, instant-killing any Space Marine IC without him standing even a remote chance. You put him in the front of your army, and made him an obvious target...He's actually daring your opponent to try to assault your Necrons, just so he can show those assault troops how it's done.
Regardless of what it is that's shooting him, the Deceiver is going to take a stupid amount of punishment before he goes down. Most opponents I've played tend to shoot anything that's strength 5 or higher and in range at our Golden Boy, even if it only wounds on 6's. And whatever's shooting him, isn't shooting your 'Crons. This allows them to rain death with their powerful guns in return fire. And if you're playing against an assault army, and he survives the shooting onslaught, he can continue this role by standing between the assault troops and your precious Necrons. That squad of Stormboyz rushing headlong towards your lines isn't nearly as scary when they have to get through the Deceiver before they can do any damage to your real troops.
Secondarily, the Deceiver can also be useful in damage control. That is, he can throw around Morale and Pinning checks at will to your enemy's units. Fearless? Don't care, you take 'em anyhow. Granted, most fearless units are Ld 10, so it doesn't matter much, but people tend to get really annoyed when you make their Khorne Berzerkers take a pinning check.
If there's a single unit that you need to make sure simply can't do anything for the next turn, the Deceiver's your best bet. I used to think that the Morale check was best most of the time, because it caused your enemy to move away from you rather than just sit still, but I've come to see the value of the Pinning check as well. Morale is best against non-fearless assault troops - it gives you valuable ground, probably allowing an extra round of shooting (if not two!) before they get back in the fight.
Note that I said non-fearless; when the Deceiver forces a morale check on a fearless unit, and they fail, they don't run away. Instead, according to the FAQ, they take armor saves for each point they failed by (as if they had lost an assault by that amount). Weird, and not nearly as effective as one would have hoped, but whatever. Therefore, it's better to pin these guys, just keeping them sitting still. It's also better to pin Space Marines in general: with And They Shall Know No Fear, morale checks have almost no real effectiveness against them, unless they're right on the edge of the board, destroying them.
Also, don't forget the feints you can pull with Grand Illusion. You want to be going first anyways, but this ability seals the deal. If you go first, you can put units on a flank, seeming easy targets, and allow your opponent to respond to this by sending a unit or two to deal with it. Then, before the game begins, you can redeploy those units back with your main force, essentially neutralizing a small part of your enemy's army before the game even begins. Another powerful use is in the Dawn of War deployment type. If you're fighting an assault army, you may be afraid of how close they can start to you - 18" from your closest unit seems to be way too close, especially since you can't bring your full firepower to bear for a full turn. The Deceiver can solve this problem: when you deploy first, rank up one squad of your warriors in a line, with maximum coherency distance, along the 24" mark. If you can get them to cover a decent portion of the middle of the board, your opponent will only have the initial 6" along his board edge to deploy on - not a lot of room for getting objectives or for getting close to you. Then, use Grand Illusion to bring your Warriors back to a more reasonable position, and watch the tears streak down your opponent's face! Yes, it's a dirty trick, but let's face it, Necron players need all the help they can get right now.
Well, it's been a fun chat, and I'll post again next week!