Writing this from the Park Hyatt BA right now as I'm sadly packing to go back to cold NYC.

We were here Thurs and Friday nights. Here are my recs
Thursday-
Dinner- La Cabrera- a parilla institution and way more popular than when we went two years ago. They even opened up a satellite location down the block to satisfy the excess demand. We didn't have reservations and when we went at 10:45pm, we had to wait about an hour for a table. We killed time at the reggae bar across the street, which has happy hour 2 for 1 drinks (I'm a sucker for the BOGO).
Once in, we ordered the mollejas (sweetbreads) which are legendary. I'm not a huge sweetbreads person, but these ones are huge, crispy and just really delicious. Ojo de bife steaks were good as well. Wasn't that expensive- came to $60 USD total, though we didn't have wine, because we were..uh... tipsy from our 3 hours of pre-dinner drinks
Going out- Glam, which opens at 1am. We got there around 1:30 and it was just starting to pick up. We met a ton of great people and had fun until the wee hours. Definitely recommend- a 70 peso ($18) cover and two vodka-energy drinks were 90 pesos ($22.50).

Friday-
Dinner at Don Julio- another parilla in Palermo Soho. It was also packed and we had to wait about 35 minutes when we went at 10pm. I like the ambiance a lot better and I thought the steaks were better as well.
After dinner drinks- Sitges- 35 peso cover ($9) gets you open bar (certain well drinks/draft beer) until 5am. Quite the steal! It was a mixed crowd- met a lot of interesting people there- a lot of Brazilians, Argentines, Aussie and European tourists. Our group ended up going to Amerika , but we called it quits around 330am. Apparently Amerika is as popular ever- with a straight section and a very active darkroom.

Overall, I love BA and can't wait to come back. At some point in time, I'd love to spend Jan/Feb here and rent an apartment in Palermo Soho.

We also did Rio/Montevideo/Punta del Este on this trip. Will update those threads when I have the time.