I've only ever been a so-so fan of the Coen Brothers. Whilst I liked its dark edge, "Fargo" I found only funny in patches. On the opposite side, I quite enjoyed "The Big Lebowski" and loved it even more after numerous viewings with its great characters and just 'out there' moments.
That said 'O Bro' lies in an entirely different direction - its not hardcore dark comedy or far out eccentric. Rather its a musical with some odd elements and on that level it works with some really catchy tunes befitting the time & place its set in. The credits say its based on Homer's Odyssey tale though only two segments (Cyclops, Sirens) are really used. Nevertheless there's moments that are touched with that very weird Coen style - the best example being a Ku Klux Klan meeting that looks like its about to break into a kick line.
On the downside the humour is lacking, especially toward the beginning when they try several gags which just fail completely (the packed theaterette I was in remained silent pretty much throughout the movie). The songs may be catchy, but many because they're so annoyingly repetitive (eg. The Sirens who just repeat one chorus over and over). Americans won't notice it, but international audiences will have trouble deciphering some of the accents in this which are very strong.
Of the actors Clooney fares the best though Tim Nelson as the totally stupid 'Delmar' is quite likeable. There's also a subplot involving the local Governor which is well played and fits in at the end effectively. The cinematography is perfect with lighting and atmosphere really befitting the sunburnt Depression-era area. Really in terms of technical merit this is a superbly crafted movie without any real weaknesses.
That said the subject matter will affect one's level of entertainment. American History in the 30's and chain gangs were things I've never been interested in - yet I found this quite enjoyable. Those who do like that field of study, or really get into the Coens brothers work will really get a kick out of this. Its not up there with 'Lebowski', but its still a darn good tale.