Casa de Pico isn't the best Mexican food in San Diego and it is very tourisity, but it still has a lot to offer. If you are in Old Town it is worth stopping by. Be aware the wait for a table be very long on the weekends.
The food is good but not great. However, the real reason to visit Casa de Pico is the atmosphere. You go to sit on the patio and sip a margarita and listen to the mariachi band.