I've never had a bad meal at Lang Van. In fact, every time I go, I'm amazed by the authenticity of the food and the quality of the service. The owner will remember you after one visit so be prepared to have her greet you at the door. The entire staff is extremely friendly and attentive. The lemongrass tofu is my favorite meal but I usually let the owner suggest a meal, which never fails.
Don't let the exterior or location scare you off. It's definitely not pretty inside or outside, but it's very clean. If you love authentic Vietnamese, it's worth sacrificing ambiance.