It's got a fantastic set up for a Western: Katie Elder is dead and her four sons arrive for the funeral, and try to make it up to her high standards by trying to avenge the death of... their father. Okay maybe the set up isn't so great... they should be trying to avenge the death of Katie Elder shouldn't they instead of some random dad?
But it gives John Wayne a great role as her gunslinging eldest son, and a nice support part for Dean Martin as a gambling son. The charisma of these two overshadows the others: Earl Holliman, in admittedly the dullest part as the dull son, and Michael Anderson Jnr as the kid they want to send to college. Tommy Kirk was meant to play Anderson's role until he got busted for drugs and it's a shame - he would have been a lot better.
The movie is never quite as good as you think it's going to be. It's got that great 60s colour, a memorable introduction from Wayne (standing among some boulders), a decent amount of action, Martin is given a scene to have some schtick, and Dennis Hopper livens up the support cast as a coward. But the villains are too weak - you never think for one moment George Kennedy is going to be a threat, and I can't remember who the other baddies are. It's confused by having the brothers reunite for their mother's funeral but avenge their father, and you never get the sense the four are brothers - they have no chemistry - and Holliman and Anderson are weak. Also the love story is under-developed.
It's a movie you watch a go "you know they should remake this and do it right." They didn't with Four Brothers.