The short version:
Comparing the SF Chronicle's review of Tres Agaves when it first opened with my experience last night, I'm sad to say it seems like they're already starting to mainstream their food.
The Chronicle said that the food there was so spicy one needed an asbestos tongue. Last night, I didn't taste anything particularly spicy, and none of the other 9 people seemed to be reaching for their water glasses either.
The online menu has interesting things like goat, crab, huitlacoche; the menu last night didn't have any of those. Most of the dishes we had were fancied up versions of things you find at Chevy's. Granted, Tres Agaves is way more tasty than Chevy's, but not necessarily any more inspired.
Except for their stellar margaritas, nice space, and fancied up the food, I feel like I could have eaten just as well in the Mission.
Mexican restaurants that have impressd me more in recent months include Tamal and Colibri. Tres Agaves is nice and I wouldn't hesitate to go back if I were in the area for some reason, but I probably won't go out of my way to arrange a big group dinner there. Especially because it's so noisy in the restaurant we all had to shout at each other.
Still, $30/pp after tax and tip, including almost two margaritas per person and shared desserts isn't bad at all for San Francisco.