La Frontera is choice #2 for me and the missus when we need a fix of street food. Street food is supposed to be simple and the tacos at La Frontera are just that, simple and to the point.
If you're looking for a Mission style burrito, you will be disappointed. But if you want tasty tacos de chicharron or carne asada, you're in for a treat.
Part of the problem here in Arizona is that we have a mix of expectations when it comes to food because there's not much tradition and a healthy influx of people from other areas here. While I'm not sold on the Sonoran Hot Dogs at many other places around here, La Frontera's sister cart (same parking lot) does a great job putting together awesome hot dogs with everything on them.
As for tacos, skip the Al Pastor, I still haven't found a place here that makes 'em the way I like (crazy thin sliced off the spit, sweet and succulent, with pineapple). Go for the Carne Asada, Chicharron or the Lengua, anything else and your guess is good as mine as we've been just twice.