Like the other poster, we found this place via a Yelp "Nearby" search and were very pleasantly surprised.
This particular branch of this restaurant is tucked into the corner of a mostly deserted shopping center in a rundown part of Mesa and doesn't look like much from outside.
Inside is a different story. The restaurant is fairly humble but immaculately clean and bright with a pleasant enough ambiance and the service was friendly and quick.
We had a ceviche salad ($12.95) and a pupusa combination plate ($7.95). Both were quite delicious and portions were huge.
If you aren't familiar with Salvadorean food, I'd recommend you start with the pupusa with loroco (an edible flower), which is the classic pupusa and extremely delicious.