The Coen brothers are great film-makers, and I'm sure it will be good. They have made some of my favorite movies over the years, from Raising Arizona and O' Brother Where Art Thou, to The Big Lebowski and No Country for Old Men.
True Grit was adapted from a novel in the first place, and their take isn't meant to be an exact remake of the movie John Wayne was in.
Ethan Coen had this to say about their adaptation of the novel.
It's partly a question of point-of-view. The book is entirely in the voice of the 14-year-old girl. That sort of tips the feeling of it over a certain way. I think [the book is] much funnier than the movie was so I think, unfortunately, they lost a lot of humour in both the situations and in her voice. It also ends differently than the movie did. You see the main character — the little girl — 25 years later when she's an adult. Another way in which it's a little bit different from the movie — and maybe this is just because of the time the movie was made — is that it's a lot tougher and more violent than the movie reflects. Which is part of what's interesting about it