The plot is more convoluted than the first two films, and there's an uncomfortable amount of exposition in the first 20 minutes, about how there's a plot by some cartel dudes to kill the president of Mexico in a military coup, and a counter-plot by the CIA to off the leader of the coup after he's killed the president. But that's by the by, it's just setting up a load of betrayals and counter-betrayals down the line.
What you really need to know is that Antonio Banderas is wandering around with a sawn off shotgun, tortured by his past, and killing anyone remotely responsible for this. Johnny Depp is a really smart-arse amoral CIA guy, who gets betrayed, and spends the latter half of the movie seeking vengeance for it, despite a serious handicap. He gets *so* much cooler at this point. The whole thing works itself out in a satisfying enough way, and there's a lot of bodies in the street by the end. Claret by the bucketful.
It doesn't entirely make sense, and I don't think it's as good as Desperado. However, I don't think many things are as good as Desperado, so no real criticism there.
I'll give it 8/10, and declare it My Kind Of Movie.