Personally, I liked "O Brother," but I'm kind of a Coen fanboy anyway.
Truthfully, parts of the movie seemed just a tad forced...understandable for a musical tho.
Here are some examples of the Odyssey thematics at work (not in any particular order):
Clooney = Odyssus
Blind prophet = Blind guy on railroad tracks
Lawman = Hades
Lotus eaters = Christians baptizing
Circe/Sirens = 3 women at the river
Turning into a toad = Turning men into swine
Salesman (John Goodman) = Cyclops (gotta love the camera angles that made him look like a "giant")
Escaping the Cyclops under sheep = Escaping the Salesman under sheets (KKK robes)
Suitors trying to supplant Odyssus = "Bona fide" slimebag horning in on Clooney's wife
Masquerading as old man/slaying the suitors = Masquerading on stage/ captivating the audience/slaying the candidate (and "bona fide" slimebag)
Redemption through entreaty to Poseidon, water carries Odyssus home on boat wreckage = Entreaty to God, saves Clooney, carried out on home wreckage