the steak is good there. not sure if argentina is the home for the best steak in the world. in my opinion, it's an exaggeration. you'll find there is alot of that here, except for the chicas who are ranked by many polls to be in the top 5 in the world. i'll take a cajun ribeye from mortens anyday.
La Cabrera is very good, but depending on your bubget you can get good steak almost anywhere in BsAs.
One of my favourites is Aporto on Scalabrini Ortiz, although it does not have the ambiance.
This all depends on what you want, romantic setting, or the best food. Personally I always seek out the places that the locals eat at. you pay less and the quality is always good.
Finally if you go to La Cabrera try to be early or book in advance, as it is very popular, ie busy.
I used to recommend La Cabrera as the best steak restaurant in Buenos Aires but not any more as it has gone downhill fast . It suffers from grandiose behaviour now with outrageous pricing and lower quality meat. The last three times I was there it was a disappointment so it was not an off night. The meat which was so tender and flavoursome is no more. The portion sizes have been reduced by 200 grams.
The biggest disappointment is the outrageous prices for lower quality and cubierto which is 12 pesos per person. In the older days there was a 10 percent discount for cash but now that is gone. To give you an idea of the rip off it has become yesterday I ate a very mediocre bife de chorizo with a friend and we had 2 soft drinks and a small plate of olives. This meal came to 128 pesos and on the bottom of the bill there was a message in big writing that tip was not included. This to me was the last straw and I will never return again on my own undertaking.
La Cabrera AVOID: There are many great parrillas with better meat at less than half the price.
I agree with Pericles -- the first time we went to La Cabrera was a couple of years ago and the meat was spectacular and the price even better! Unfortunately it's all too common with places that get popular, the prices rise and rise and rise and the servings get smaller etc. I still prefer going to local cantinas over big name restaurants, you can get great food at much more reasonable prices. Los Chanchitos used to be really good, I haven't been over there in a long time though. They are owned by the same group as Ale Ale, which is also supposed to be good.