Mad Mex fills the much-needed niche of "popular hangout spot that also has vegan food available." It's a great place to take your non-vegetarian friends since it has something for everyone.
That said, I was somewhat disappointed with the texture of the soy cheese (like little sticks of cardboard that were flavored like cheese) and the tofu sour cream (think pureed tofu with lemon). I would stick to the menu items that don't rely on cheese, like the burritos or soups. One major bonus: there are sopapillas for dessert (which the waiter assured me were vegan, but you might want to check before ordering).
All in all, Mad Mex has made a laudable effort to extend their menu to vegetarians and vegans, so it's worth going just to show appreciation for their efforts (and perhaps to subtly slip the contact information of Follow Your Heart Soy Cheese and Tofutti Sour Cream).