The restaurant is very near downtown - if you are near the subway B line, just take it to the direction "J.M. de Rosas", get out in the Callao station and walk two more blocks.
It's an all-you-can-eat (at dinner it's 73 pesos), with salads, hot dishes and desserts (try the vegan ice cream). The hot dishes are somewhat greasy and there's too much fried stuff. The salad variety is good though - you can actually eat everything a little from it and you'll be fine.
There is also a takeaway possibility, charged by kilo.
The ambience is very nice but be prepared to speak some Spanish, as the owner is Chinese and the waiters are Argentinian...Pros: central location, good food, good value
Cons: too much fried/greasy stuff