I went here for the first time today during the lunch hour, and holy moly was it good! A friend recommended it, so we all went (as a rather large group) to try it out.
It's not a very fancy place, but boy does it deliver some great food! And the price is unbeatable; just about everything is under $10. I had "Pepe's Sample Platter" for ~$8 so I could try a little bit of everything. Others got burros, enchiladas and I think someone even got a tamale. Not one of us was disappointed.
The service is also extremely fast and very friendly. If you're looking for a place to grab a quick bite to eat south of Old Town, skip the fast food burgers and stop by El Molino.