OK. I'm back.
So in the meanwhile, a lot of people have conveyed the game's darkness. It is true: played straight according to the rulebook, no game approaches this one for bleak. For that reason, it can excel as a one-shot, where you are willing to pull out all the stops and have fun being miserable before you're eviscerated.
For a campaign, the advent of the supernatural and the dawning of "the Truth" has to be much more subtle and slow-burn. In this way, it can/should be played much like Call of Cthulu or perhaps an X-Files-esque investigation game. (I personally draw most of my inspiration from the TV show Millennium).
The rules vary slightly in the 3 editions of the game. I personally recommend Target Games' 2nd edition rules, not the overpriced 3rd editions that are out now. While out of print, the 2nd edition isn't too hard to find used. In all cases, the RAW are a little blah. They do feel a little "d20 lite." I've toyed with modifying the rules or even using another system altogether, but aside from my own laziness, I gave up on that because I realized that the "blah-ness" of the rules actually allows one to focus on the story, which this game really requires.
Combat is extremely deadly. Most characters, regardless of combat skill, will not likely survive two or three fights without healing (i.e., days to recover) against competent opponents. Even crazy street people with dirty knives (a fave violent encounter in my games) can kill a PC. Once bonafide "monsters" appear, the PCs better be in the mood to run (if they can, that is).
To be honest, the game was definitely too dark for my players. They embraced a series of one-shots over the years for my sake, but when I proposed it as campaign material, they told me that I had to lighten it up.
Without getting into too many details, my cosmology re-write offends the hardcore Kult players (many of whom dwell on rpg.net and on the kult boards). Basically, God (who in my game is fundamentally a benevolent God, not a selfish detached being) has disappeared, but for good reason: the modern world's crisis of faith and belief has driven him to exile (in other words, God essentially says, "If you don't need me, then I'll go"). As a result, the world gets increasingly horrific as humankind tries to live without God (oh, I did mention I was correcting Bible tests earlier, right? My gaming group is entirely made up of religious people, so I can get away with storylines like this). This re-write provides hope for the world, as a good God is out there. He's just WAY out there!
As for your other questions, tylermalan, yes, the story is great. Yes, the themes in the game are very mature and mechanically built into the system (it's not out of the question that your PC is a drug addict, a pedophile, or a schizophrenic). It tends to vacilate wildly between subtle, weird psychological horror and totally in-your-face splatterpunk (I personally like the dichotomy). The Clive Barker comparison is totally apt. The Silent Hill games and movie also fire me up for Kult.
That should get you started. Any other concrete details I can provide?
C