Yes, Twain is successful at rendering the Southern dialect. The beauty of Twain's use in dialogue is that he uses just enough to make his writing authentic, but not so much as to confuse the reader. Other novels by authors dealing with the South or other geographic regions sometimes over-utilize the device of dialect, thereby alienating readers at large.
Twain, however, masterfully crafts the language in such a way that the dialect is a positive supplement rather than a detractor. Stories like Huck Finn and Tom Sawyer would feel empty and lifeless without the use of appropriate dialect.
Twain has always used heavy dialect to capture the essence of his characters in his novels. The dialect creates a sense of setting and intellect of the characters. The advantage of this is for the reader to obtain that feeling of the setting, which in this case is the South during that time period. It also allows the reader to grasp the amount of education that the characters have and to feel a certain amount of connection to them. Dialect is a great way of creating imagery in a story.
One of the disadvantages would be again to the reader. If the reader cannot understand the dialect or make out what the author is trying get across, then some of the story can be lost on the reader.
YES THE BUTHOLE DIALECT IS AMAZING FOR THE BUT SEX MARK TWAIN HAS EVERY NIIGHT