Stumbled upon Ay Caramba when meandering around Harwich tonight looking for a chowish place to eat, and was drawn by a sign promising a "truly authentic Mexican" meal. I can't speak to authenticity, but I can report that the meal was outstanding on all counts.
I ordered a salad special, mango and jicama tossed with a honey-lime vinaigrette on some lettuce, and a burrito with pork cooked in a mild chile verde sauce. Both were delicious. The mango was fresh and perfectly ripe, the jicama was crispy, and the salad wasn't over-dressed. The pork in the burrito tasted like it had been barbecued, with even a few pieces of the edge of the pork mixed in (my favorite part). The interior pieces just melted. The burrito was absolutely huge, two meals' worth for me.
They also have a number of authentic (this I can vouch for) drink options, including some agua frescas, mexican hot chocolate, and Jarrito sodas. They serve chips with two homemade salsas, a delicious tomato salsa and a so-so salsa verde.
Service was solid. I requested no dairy on the burrito and it wasn't a problem. Waitress did seem like she was ready for her shift to end, though.
Total for salad + burrito + a tamarind soda was $21 including tax and tip.
For Boston-area hounds, I'd rate Ay Caramba just a tick below Ole Mexican Grill in Arlington. And that's meant to be high praise.