The first one we tried was Torchy's Tacos. The tacos were gourmet and the queso was excellent, but don't expect fast service. That is just part of South Austin culture.
Our favorite mobile food vendor was Franklin Barbeque. We got to talk to the owner for a long time and he told us how he started by using friends as guinea pigs to try all of his recipes. Pretty soon, his backyard bar-b-que parties got so big that he had to get parking permits. He bought an old trailer and a smoker, fixed them up himself, and had his dad help him make picnic tables. Now he has a line of sometimes up to 60 people waiting to get his bar-b-que. You have to be quick because he is only open from 11 until the food is gone. Some of the best bar-b-que and sauce we have ever tried!