I agree with most of the comments already posted: it's a good Western, with Mexican Lancaster starting off as apparently almost servile in front of the Yankee Tanner, but then showing he's made of steel. Whe he dons his old army uniform it's a bit like Clark Kent transforming into Superman.

The ending was a bit of a letdown, and criticisms already posted give a rough idea of what it was. I didn't find it very convincing, and I'm grateful to rrichr for pointing out that heavy-handed editing leading to the cutting of an earlier scene didn't help:

"Segundo has entered Tanner's parlor to inform him that a certain Bob Valdez desires an audience ... Tanner turns to Segundo and smirks, 'I don't know any BOBE Valdez', mocking Segundo's densely-accented English. For just an instant, just a blink, Segundo considers putting a .44 pill in Tanner and high-tailing it back to Mexico. Then he lets it go."

There aretimes, such as this one, when I think that those who edit films should be test-run before a small audience, if only to see how far deletions can be unsatisfactory.