You know when a movie has its big "sad" moment? The string/piano music swells, and the actors have tears pouring down their face, and they give some powerful monologue about loss, or despair, or hopelessness? When it's done well, you feel the weight of the moment. But, then, its just a moment. The moment ends, and the effect fades.
This isn't that.
This isn't a sad movie. This is a melancholy movie. This is a movie that never has its operatic sad moment; instead, it moves slowly and subtly. Oscar Isaac gives what is legitimately one of the best performances I've ever seen, as a man who, quite simply, fails. For almost two hours, he fails, and he never once wins. Despite this, he never breaks down, or cries, or tells anyone about how miserable he is, like one might expect in a movie like this; after every failed Sisyphean task he just tries again, and it's heartbreaking and beautiful and sticks with you in a profound way, never letting go. Life breaks Llewyn Davis, the character, in the same way Llewyn Davis, the movie, breaks me.
This movie, about the tragedy of perseverance, is excellent, but, I suspect, not for everyone. If you think you can handle a movie where the main character stands in the way of his own success, where he is robbed of any satisfaction in part because of things outside of his control, in part because of things entirely within his control, then watching this movie is a must.
Also, you must be able to like/tolerate folk music.