Robert E. Lee Clayton was....

Very true. I think Brando did a superb job, he took what could have been a run-of-the-mill western and made it into a colourful, different and thoroughly entertaining piece of film that was a cut above the rest. Jack Nicholson and Harry Dean Stanton were brilliant in their roles too, but there is no way that Brando ruined this film.

To me, it has an uneven quality like you said, but in a strange way that imbalance fits. Because the film can really be divided into three categories of character outlooks: You have the landowner and his family, who are upright, self-righteous and pristine, you have Jack and his crew of thieves, who are amoral, jokey and very rowdy, and you have Brando, whose character seems to personify total and complete mental illness :). Its a strange messed up film, thats why I love it.