We went with another couple to The Mission and celebrated our anniversary. It was dark inside but wasn't stuffy or creepy. Our waiter was relaxed and helpful. He suggested the avocado margarita. Give it a shot--you just may want a refill (I did!).
We started with the delicious guac and shared a few small plates (our waiter was spot-on with his recommendations re: quantity). By far the best part of the night was the pork shoulder tacos. They were make your own with falling off the bone meat. We finished with pumpkin bread pudding.
Never rushed or harried, we had a great time and recommend going with a few people you can sample the dishes with. Make a reservation if you go during peak weekend hours.