Fiesta is definitely my favorite of the fast food Mexican places in town. I actually eat their food sober, although after a night in old town the burritos are even tastier, if that's possible.
I usually get a breakfast burrito with ham, eggs, cheese and pico. Their carne asada is pretty good too, but it tastes more like ground beef than steak to me. I like to add rice and get a side of sour cream, and for a mere $5 they are definitely filling.
My only gripe is that Fiesta isn't open 24/7 or offer the late night hours that some of the other -beto's do (they're open til 3am on Friday which is great, but 2am on Saturdays and 1am during the week). I work at a bar and sometimes don't get off work until 2:30 or 3am and am disappointed when Fiesta is closed "early".