This was my favorite budget restaurant/café in Buenos Aires. It was located only a couple of blocks from my hotel (Hotel Principe).
The food was always good and cheap, and the service very good too. I had a peach licuado (smoothie) for 3.5 pesos and pasta with a veggie sauce for 6 pesos. The whole tab was about US$3.50 for an excellent meal. Just about anything you could want is on the menu, including pastas with 12 different sauces to choose from, pizzas, sandwiches, milanesa (thin chicken-fried steak), desserts, smoothies in 8 flavors, etc. If you just want a light meal, try a fruit licuado with a ham and cheese tostado (a toasted ham and cheese sandwich on thin white bread).
It's right on Avenida Santa Fe and I never saw the "closed" sign on the front door. However, I went there at 5pm one day and found that the kitchen is closed around that time. It's just too early to consider eating dinner in Buenos Aires!
Price update 2006: prices have gone up by about 2 pesos per item, so a meal will be closer to US$5.00 now.