Warlocks are especially not-squishy when Demonology specced. here's why...
For the sake of things let's say Bob the level 60 Warlock has 5500 HP. Based on profiles here on Allakhazam, many warlocks have more than 6000. :o
Soul Link, the final tier Demonology talent, directs 30% of physical damage taken to the pet. When fighting melee classes like rogues or warriors in PVP, this is essentially a 30% increase in total health, because most if not all damage that you take will be physical.
30% of 5500 = 1650. Now we're at 7150 hypothetical HP (until the pet dies, of course). Note that this applies no matter what pet the warlock has active.
Now, say Bob is using his Voidwalker. The Master Demonologist talent reduces all damage taken by the warlock by 10% while VW is out. so we're at a total of 40% damage reduction.
40% of 5500 = 2200. This brings it to 7700 health. Quite a bit ^^
Also remember that Demonology warlocks have instant summon, so there's nothing stopping them from sacrificing a Voidwalker for 30 seconds of invulnerability, and then immediately insta-summoning another pet: even another Voidwalker for another 30 seconds of the sacrifice shield.
Truly very difficult toons to take down :)