There are two Cancun locations -- one in the Mission near 20th and Mission Street and another at 6th and Market downtown. Both serve the same fantastic Mexican food at rock-bottom prices. For less than five dollars you can wrap your hands around a super burrito with your choice of meat and a basket of chips with homemade salsas. The pollo asado (grilled chicken) is highly recommended in taco, burrito, or quesadilla form.
San Francisco's Mission District routinely gets cited as having the best Mexican food in the country, and Cancun is one of the places that earned that reputation.