I think it goes without saying that Mark Twain is a genius, as a man, humourist, and writer in general. The characters he created in 'The Adventures of Tom Sawyer,' and more importantly, this book, are classics in literature. I loved this book so much - Twain's style is entrancing, and makes you forget at some points that you're actually reading a book. Reading 'Huckleberry Finn' is an experience, one that everyone should have.
It deals with everything American: the racial divide, the search for identity, the Actual and the Imaginary, the power of the individual over the many. The list could go on. Twain wrote a deceptively complex book, because children have loved it, and university students will continue to study it. It's a picaresque novel, a Bildungsroman, and more. I have heard about lots of controversy concerning racial language, but I stand firm in thinking that it's part of the journey, and should be there in order to adequately look at the world Twain has created.
The relationship between Huckleberry and Jim is really heart-warming: they're two outsiders trying to find their way in the world, and it's beautiful how they connect, and even more beautiful when Huck realizes just how unimportant race in defining a man. Jim is wise to more practical things (despite his superstitions), and it rubs off on his new friend as they drift down the Mississippi River.
If I can find any fault, it's this: the book goes on longer than it should. While it's nice that Jim is rescued in the end, and you get some kind of closure, I'm inclined to agree with Hemingway and other critics that it should have ended when he was handed over to the Phelps family. That is where Huckleberry's story really comes to a close - Tom Sawyer shouldn't have come back into the novel.
But that's minute. Read 'The Adventures of Huckleberry Finn'! It'll stay with you for a long time after you've done it.