Carolina's is an icon in the Phoenix Mexican food scene. Otherwise, they would be hurt by their terrible location, old worn out building, and lack of accommodations for groups larger than 4. Having said that, the food obviously matters above all because there is a constant flow of people from all classes of life walking through their door.
I've tried the enchiladas, tacos, and tostada (all excellent). I even purchased a dozen flour tortillas to go and enjoyed a piece of Carolina's at home the next week.
Don't expect to sit down and be served but you can expect great food and a piece of Phoenix history. I recommend phoning in the order and picking up. I see a lot of people walking in and grabbing their phone order to go.