Today my husband had his interview. He entered the Embassy at 9:30 AM and walked out around 11:00.
I didn't go in with him, but he told me that everyone else in there for similar interviews had gone with their spouses. So apparently right now at the US Embassy in Buenos Aires that's allowed.
I had downloaded the list of documents that the Embassy here says you need and organized them in the order they request. So he told me it was really easy to just hand her everything one at a time, as she asked for each doc. One thing that stuck out to him was that she asked everyone for evidence pictures (of the relationship), but no other couples had brought them. He had them and it just made it go smoother.
When he got to the interview part, he was only asked 5 or 6 questions before she voided his tourist visa and told him that he would be receiving the immigrant visa. We've been married for almost 9 years, so I think that might have something to do with the quick interview. Great experience over all!