I've been a big fan of phat burrito since i moved here about four years ago. the first thing i'll say is don't let the outside scare you. the funky paint and the bars on the windows are from when the southend was more run down.
the second would be to try the bbq chicken tacos. i know it sounds strange, but they are very addictive. i used to call in just to make sure they had enough as they make it fresh and once they're out, they're out. i've never really been there when the place was packed. so, the atmosphere is very laid back and if you have a business card, feel free to slide one under the glass on their tables. this place is great when checking out the shops and galleries of the south end. so, give it a try and have a few tacos on their patio.