Bio is a small and welcoming restaurant with room for maybe 30 inside and several tables outside on the sidewalk if the weather is nice. It's all-organic and all-vegetarian, they do have dairy cheese and ice cream but also have a few dishes with vegan cheese and ice cream alternatives. About 60-70% of the whole menu is or can be made vegan so there are plenty of choices. They also have about 10 raw dishes so if you're looking for high-quality food in Buenos Aires, this is an excellent resource. In addition to their main purpose as a restaurant, they also have some organic food products for sale as well as cooking classes and do catering. The menu is in Spanish and English and I believe some of the staff speak English.
It is a hard area to park on the street in, they do offer validation for one hour at one parking lot so if you're planning to pay for parking check which lot first. They accept reservations and I recommend calling ahead if going at a popular time like weekend dinner as the place is fairly small and does fill up.
In the evening, they light candles and it's a fun environment; I've seen a number of birthday dinners there. When you're seated, they'll bring you bread and a (vegan) spread complimentary starter. I recommend the rice empanadas which get a tiny bit crunchy and were excellent -- the filling varies but I believe is always vegan. In my experience all the dishes are good and filling but a few seem to be rather salty so ask if you're sensitive to that. Be sure to save room for dessert! They also have a limited wine and beer selection and even a few vodka drinks, as well as natural fruit juices. I haven't been for breakfast or lunch but they do have offerings for all times of day.
Cons: Not so easy to get there/park, Might require reservation