I caught Once Upon a Time in Mexico last night, and I thought it was the better than Desperado (haven't seen El Mariachi yet). Everything in this movie was bigger and better than anything that came before it in the franchise with the exception of Antonio Banderas' part. And, of course, if you despise Banderas (the way I do) then you're certainly glad when the movie shifts to Johnny Depp's (who steals the movie as always), Mickey Rourke's, Wilem Dafoe's, or Ruben Blades' point of view. IMO, the constant shifting and sub plots upon sub plots keep the movie from turning into the snooze fest that was the last forty minutes of Desperado.
I would give a longer, more in depth and professional review...if I weren't such a bastard. At the moment, you'll have to settle for a, "if you liked Desperado you'll love this one". Good script, great cast, great action, and a lot more ambitious than any of the previous movies in the franchise.