B13 is absolutely my favorite Bar in Madrid. I've been there very often with all my friends (omnis and vegetariens) and everyone loved it! There is a cool atmosphere, the staff is super friendly, it is always crouded with nice people and the food is awesome. They mainly offer vegan fast food. Everything is very cheap, especially the drinks. They also have cake and I can definitly recommend the Tortilla!
If I could, I would give 6 stars :-)