It's a seminal classic. It's often referred to as being one of the "the great American novels" because it captures the zeitgeist of the times. Twain effectively changed the south American vernacular into clumsy speaking to an art, and it's virtually always understandable (the book I have came with footnotes, explaining words I didn't immediately get - I'm British).
The book is well-written, one of Mark Twain's finest writings. He once judged his own work and said Joan of Arc was his best (um... respectfully, no sir). The book is witty, and Tom is the archtype of a mischievous kid. He annoyed me at quite a few times during the book, reminding me of a sibling that would always do things I'd get into trouble for and get away scot-free. But it's a really good book, is one of two books without any consistent plot (the other being Catcher in the Rye) that I enjoyed regardless.
If you read it for any reason, read it because it's influenced so many people and - if anything - it may do the same for you. Twain has a way with word, as you'll see.