There are several churrasco a kilo restaurants along Avenida Nossa Senhora de Copacabana in Copacabana. This particular one is always popular as it is cheap and has a great barbeque meat section at the back. Desserts are not as good as others I've visited along the same street.
Definitely some of my best memories of Rio de Janeiro comes from eating in the restaurants per kilo (restaurante a quilo) in Rio de Janeiro. You just go, grab your dish, serve yourself the food of your election, collect the type of grilled meat that you want and pay accordingly to the weight of the dish.
In Grill Inn you will not only find a huge variety of (delicious) salads, rices, salted complements, fruits, desserts and even Japanese food, but also a fantastic grilled meat. At the end, after paying and before leaving, you can take some free coffee, tea or mint liquor, from the place next to the entrance door.
As for prices, although it is a good one (10-15 reais on average, like US$4), it is more expensive than other restaurants a quilo in Rio (you may pay 6-10 reais). However, I think it's well paid. I loved it!
Hint: Try to be a bit early, though, especially for lunch, because at that time there are lines of people waiting for space.
Favorite Dish: The grilled meat!!!!!!! I also loved the juices (sucos), especially that of strawberry (suco de morango)