Ghouls have been one of my most beloved features of the Death Knight for some now. I like having an undead squire by my side, and they can be pretty useful for some extra dps and even some lifesaving interference in a pinch. However, they also felt like one of the least firmed up aspects of a Death Knight for some time now, with abilities coming and going on a patchly basis, and bugs that kept the ghoul from being summoned or allowed to summon a massive army of ghouls in no time flat.
Still, they have their uses, and despite still being incredibly flimsy healthwise, do offer a decent amount of extra damage when they're up. The drawback is that 50 silver is a lot for a pet that lasts less than 5 minutes, and without points in the Unholy tree, the Ghoul is completely uncontrollable, and may have a tendency to run into suicidal situations without listening to a word you say.
As far as perfecting the Ghoul and deciding on its place in the class, there seems to be two schools of thought. The first, mostly made of Unholy specced Death Knights, sees the Ghoul as a fun and useful part of the class, and wants to see it tweaked to have good survivability, good dps, and generally be worth keeping out. The other class see Ghouls as mostly a novelty, a thing to be bought out occasionally, and such a pain to upkeep that they don't really want to have to deal with it as a pillar of the class. They'd rather be able to forget it exists when they want to.
Both positions have their passionate supporters, but Blizzard seems to have found a rather ingenious way to give both sides what they want: Talents. Recent talent changes provide a lot of buffs, and if you collect them all, you should have a much easier time using your ghoul to your hearts content.