I've been following a couple of online forums and there's a discouraging recurrent theme: Buenos Aires seems to be suffering from a plague of various sorts of crime. At its most threatening, it manifests as muggings and armed robbery on the street - no area of town seems to be immune to this problem. Lesser threats include all sorts of taxi scams, bogus currency, short change artists, and pickpockets. Add to this the very real possibility of public demonstrations against the government, and it all sums up to a volatile and slightly threatening environment.
I'm not averse to going - in fact, I'm still very much looking forward to the trip. But it will be interesting to see just how vigilant we need to be, and how much that vigilance detracts from our overall enjoyment of this vacation.