While crime and poverty have grown considerably since devaluation, BA is still a reasonably safe city by world standards. I live in Recoleta, go out at night frequently and have never had a problem - other than a pickpocketing on a bus and someone's unsuccessful attempt to rob me by throwing mustard on my suit - in front of the Teatro Colon (an old trick here). There are some dark streets in the Micro Center and parts of Palermo that I do not like at night however these areas are still fairly secure. Needless to say, it is safer where there are lots of people on the streets. You might avoid dressing in ways that make you stand out (leave the baseball caps at home), keep cameras discreet. Use the same common sense you'd use at home regarding carrying money, etc.
