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I believe the Necrons are the only army that has truly stood the test of time.

They have the most solid list available; 1 Necron Lord and 20 Necron Warriors gets you started off. Saves the hassle of needing to decide what to make the core of your army from. And, they're reasonably priced for their abilities.

People complain about variety in Necron lists. The HQ and Troops selections lack variety, yes, but that's the idea. The grunts generally don't need variety. Keeps everything simple and functional. They kind of have to when they're automatons. Each other slot has 3 units to choose from, which are all immensely powerful when used properly, and it's these classes that allow you to add the variety.

It's the imagination of the owner that limits an army, and when they find a list that works, they aren't likely to change it, because they enjoy winning, and they probably don't want to risk losing for vanity's sake by changing it to suit someone else.

Yes.

Wraiths, Scarabs and Destroyers.

I love that the Lord can be mounted on a Destroyer platform too. That's excellent.

I think they viciously rend vehicles and heavily armored targets. If I recall correctly, never having owned that book before...

any use against infantry?

They're very useful against infantry. 3 wounds per base, a respectable armor save (as far as swarms go, anyway), and Fearless. They can tie up even decent infantry for a couple of turns. On top of all that, they're fast enough to get them where they need to be.

Destroyers. They're ridiculously underpriced for thier utility.