More after the break:
This ubiquitous piece of fortification, in my opinion, is almost mandatory right now, ESPECIALLY for armies that don't have their own fliers, which is almost every army I play. I look at it like this: I had a Dreadnought in my Space Marine army, with twin-twin-linked autocannons. He cost 125 points, and rarely made it through the fight. A Space Wolves Dreadnought would cost the same. And of course that is bare minimum. No drop pod, no Venerable upgrade, nothing. Just a stock Dreadnought with 2 twin-linked autocannons.
Now let's take a look at the Aegis Defense Line, with my mandatory upgrade, the Twin-linked Autocannon battery. It's 25 points less than that Dreadnought. And it can fire at Fliers at regular ballistic skill. And can fire at an enemy reserve unit in the enemy's movement phase. And it's got a toughness value which means it can't be killed in a single shot. And 2 wounds. And an armour save. And a cover save if you place it in some terrain. FOR 25 POINTS LESS. This isn't a good deal... this is worst example of GW's 'game balance' I can think of. Here we have 100 points of force multiplier. What's force multiplier (for the uninitiated)? It's something that makes the rest of your army better. You see, in addition to the items already mentioned above, it also comes with almost 3 feet of 4+ cover. Cover that gives the guys behind it a 2+ save if they go to ground. It gives vehicles behind it a 4+ cover save, because of it's height. This thing isn't amazing... IT'S INSANE!
Name me anything else in the game that for 100 points can deal with fliers, deep strikers, out-flankers (all outside of your turn no less), is still viable on your turn, gives over half of your backfield a great cover save (since most cover got nerfed to 5+) and can prevent a lot of the devastation from those pesky 'alpha strike' armies like Guard? What's that? You can't? I'll tell you why... there isn't anything. Nothing in the game gives your army so much for so little.
Here's another point to consider: You may be '100 points less' in your army, but in the Purge the Alien Scenario, you've just denied your opponent a victory point. Because the Defense Line can't be destroyed. Sure you can take out the gun, but the deal is the entire unit has to be destroyed to count as a victory point, and the Defense Line ignores that. So there's 100 points of your army that other guy can't touch. It's crazy.
And how do you work this low, low-costed miracle? Simple. Stick someone on the gun who wasn't going to fire anyways. I love my Rune Priest for this. He casts buff spells (sorry, psychic powers...) on my army, and uses his shooting phase to man the gun. If I get a flier or something in reserve, I lost my shooting phase, but he wasn't going to shoot if the gun wasn't there, so technically I've lost nothing.
Marine Chapter Masters do it better. You fire in your turn, he bring on reserve in his (1st or second, depending on drop pods), you fire in his movement phase, then in your next shooting phase, while your gun is recharging, or whatever, you drop your orbital bombardment. Chances are you're only going to shoot in one of your enemy's phases, unless the guy is a total jerk and brings a ton of fliers, so on that turn you're losing shots from the autocannon, you're still in the fight by dropping a tactical nuke on something.
Other great options for manning a Defense Line? Farseers. They typically don't shoot or fire offensive psychic powers, so they are free to man the guns. The latter half of a Devastator Squad. It's like getting 2 more heavy weapons in that squad... but better! Word to the wise though: don't let your Devastator (or Long Fang) sergeants man the gun. they have useful abilities that require them not to fire anything. Use a feeb marine instead. Or grots... who are amazing. A cheap ten-strong squad costs almost nothing, and mans that gun at BS 3, which is very hard to come by in the Ork list.
Obviously my advice is centered on the four armies I play, but the advice is universal. Get someone who otherwise isn't contributing a lot, put him on the gun, have him shoot at stuff, BAM more long-range threat for ANY army. That's the thing that just really kills me about the thing: any army can take it. Except Tyranids. Boy, GW must really, REALLY hate bugs...
*And again I steal from Google Images, but you have to admit, these are pretty cool defense lines!*