I am going to say that I just love the new Guardians. Their Battle Focus rule is just plain awesome, and if you throw them behind an aegis defense line, alongside a Warlock, expect nothing but greatness. Lets take a look.
Guardian Defenders are now BS4, and still come in mesh armour for that wonderful 5+ save. Guardian Defenders though have a good array of medium to long range weapons to choose from. Not only that, but the unit size can go from 10 to 20 which allows you an extra heavy weapon platform at 20.
So now look at your options: 2 Scatter Lasers at BS4 that can move and shoot. Thats 8 shots. Attach a squad of 20 to a Quad Gun and now you have 12 shots downfield.
Now when the enemy gets close, the Guardians behind the Aegis Defense line excel. Simply move out of it if the enemy is just beyond your range, shoot, and then run (with a re-roll because your fleet). With bladestorm, a roll of a 6 means ap2 shots that auto wound. This means that your Guardians will be extremely deadly to anyone getting too close, especially when you field 20 of them.
Why Guardians behind an Aegis Defense? They can get shrouded, and take that 4+ cover save to a quick 2+ cover save with the simple addition of a single Warlock. The Warlocks primaris power gives conceal, and I know its an 8 leadership to get it off, but that is simply an amazing power, and I think this is where the Warlock excels.
(for those that are not aware, if a single model in the unit has shrouded, the entire unit gets it)
Why 20 guys in the backfield? That is where you are placing your objectives. You need someone to hold backfield objectives in 6th edition. These guys do the trick, and do it well. Hide your Farseer in these squads, and your leadership just went from a measely 8 to 10. Its a good place for him to be hiding while he buffs others in your army with guide and prescience.