Plot/Story
The basic story is a simple revenge plot. One gang kills another gang's leader, and the other gang wants revenge. Well, at first. The plot quickly jumps into a multitude of other things, jumping between the stories of both gangs, and unraveling incredibly smoothly. It primarily focuses on the struggles of Kakihara and Ichi, who are both incredibly disturbed and perverted, but in VERY different ways. I warn you: these guys aren't like anything you've seen before. They reach a new level of sick and twisted that you've never even seen before (which I'll cover in more depth in later). It also focuses on quite a number of minor characters, some being very down to earth, and others being incredibly whacked out beyond realistic levels. But, somehow, it makes you care for all (or most) of these characters (especially Kaneko), and you're able to suspend your disbelief just enough to make these characters seem plausible. Another way the plot is great is in its excellent sence of pace. The story flows incredibly smoothly, and the bits and pieces of backstory are all perfectly placed, along with the individual character stories that perfectly tie into the main story. Oh, and I can't forget about the ending. I don't want to spoil anything, but I will say this: It will shock you, in at LEAST one way.
Characters
But, what would a plot be without a wonderful cast of characters to keep it going? Here's a basic list of the main characters:

Kakihara - a sadomasochistic yakuza who has slits in his cheeks so deep, that when he opens his mouth hes virtually a human pacman. I guess you could call him the "main" character, though he isn't much of a protagonist. Arguably the most fucked up of all the characters (next to Ichi), you'll know that you wouldn't wanta mess with this guy right from the beginning.............. but his really messed up side doesn't surface until near the end of the story. He's always unpredictable, and, surprisingly, there will actually be times when you root for him.

Ichi - the title character of this story, Ichi is a crybaby with a kick that could take out a T-Rex. He's definately not your typical hero. The things that go on in his mind will make you wonder why the hell you end up caring about him so goddamn much. Here's the reason: its because it's not his fault that his mind's like that. The only time his mind reverts to that..... stage............ is when he's remembering the people who bullied him in the past. Otherwise, he's one of those guys that you just want to chill with. Someone who'll always be nice to you and who you can value as a close friend. I guess you could (kinda) call him the tragic hero of this story. Then again, you could also give that title to Kakihara. All depends on interpretation, I guess.

There are also multiple other characters, such as Jijii, the man who is manipulating and messing with Ichi's mind, and who seems to know more about the situation than anyone else, Kaneko, and man torn between his devotion to killing Ichi and fullfilling his duty to his gang and his frequently bullied son, and the twins, two guys that are............. you know what, I'm just going to let you read about them yourself. But, I'm not going to go too deeply into all these other characters. But I assure you, each and every one will keep you engaged.
Drawing Style
Great, but not perfect. That's basically how I would describe the style. It's a very beautiful, detailed manga, but there are times that the art looks somewhat dated and/or messy. This definately isn't on the level of something like Battle Royale or Gantz, but it does have very detailed characters, and some of the most gorey/disturbing drawings you'll ever see in a manga. Be warned: this is not a manga for the faint of heart. It's sick, disturbing, and will definately make you cringe more than once. Even my second time through, there were times I was afraid to turn the page, because I just KNEW what would happen.
Enjoyment
Honestly, this will be one of the most enoyable mangas you'll ever read. Well, if you have a thing for expliotive trash, that is. It disturbed and apalled me, and yet it constantly kept me engaged in its deep plot and characters. So, if you can stomach it, I guaruntee you'll find at least SOME enoyment here,
Overall
This is a truly outstanding manga. I wonderful read from beginning to end. The plot, the characters, and the over-the-top violence and gore are some of the best ever seen in a manga. I really recommend that you read this. And hey, this manga even inspired a few big-name hollywood directors. Eli Roth is a HUGE fan of Ichi (even making the cast of Hostel watch the whole movie to get a feel for the gruesome violence), and there's a scene from Quentin Tarantino's Kill Bill that plays as a direct homage to this manga. Bottom line: Quentin likes it, so why shouldn't you?