Well, after watching both "El Mariachi and Desperado" and "Once Upon a Time in the West" and "Once Upon a Time in China", it's pretty clear what movie was going to be coming up eventually, right?
Once Upon a Time in Mexico doesn't have much to do with the first two Mariachi movies. Not that the other two have much to do with each other, but this has even less. There's a great deal of Johnny Depp in outlandish outfits talking strangely, but it's not quite as entertaining here as it is in Pirates of the Caribbean. Antonio Banderas shows up to shoot things in slow motion, but there's not as much of him as you'd think.
In fact, most of Banderas's screen time is in flashbacks, which means that they don't have to be strung together in any sort of narrative. Instead, it appears that Robert Rodriguez had a number of action scenes he wanted to include, and just put them in as dream sequences for Banderas's character to remember.
I have to say, I strongly approve of the kid selling Chicle. Every time I've been to Tijuana, I've spent most of my time hearing young children shouting "Chicle! Chicle!" For me, that's one of the sounds that says "Mexico!" loud and clear. The others are "Poppers!" and "I am so drunk! Are you in the Navy?" although that last one's probably Tijuana-specific.