I know this is going to sound like hyperbole, but this makes my list of the ten best movies EVER. What a shame it's so obscure. Highly recommended, for older kids and adults. A bit dark and scary for younger kids.
The script draws in some of the more obscure Twain works such as The Mysterious Stranger and Tom And Huck Abroad, and does so seamlessly. The claymation is excellent. It really gives you sense of both the humor, darkness, and humanity of Mark Twain. Just a great, great movie, and an excellent overview of Twain and his work.
BTW, I believe that there is a later live-action movie with the same title (The Adventures Of Mark Twain), so don't get the wrong one.