I've only had the soft tacos here but I've had them about 4 times and every time they were delicious and to my admittedly non-expert-level tastes, very authentic. The interior is nothing to speak of, and the server (owner?) I've had every time is very nice but does get over-busy often as she has to work the entire place herself.
June 2013 Update: Now that I've been about 15 times and had more tacos (still excellent) but other things too I thought I'd say a little more. The burrito is also very good. Go on Tuesdays for cheaper tacos, and Wednesdays for cheaper burritos. There are a few very good beers on tap including my favourite, Flying Monkeys IPA. The first server I had is gone and the woman who works almost every time I'm there is uniformly excellent.
A very nice, unpretentious antidote to the much-hyped come-lately, Playa Cabana, 2 blocks away.