My experience, anecdotal as it is, has been overall a good one, but I do try to take some basic precautions. I do not try to get change for a larger bill from potentially risky sources (taxi drivers, waiters street vendors), and when using a taxi I make sure it's a "Radio Taxi", which is easy as most of them are. I never use a gypsy taxi.
I did have one attempt to swindle me when my wife and daughters boarded a taxi at our hotel on our way to the London Grill (too bad, it was closed several years ago). The hotel porter gave the driver the address and the little chatting I did with my family during the ride was in English. Back then inflation was rampant and the taxis couldn't keep up with the frequent rate adjustments, so they used a chart to show what the rate was when the meter clocked a certain amount. Well, when we got to the restaurant the driver said that the fare is X which I knew was way off the mark. So that was when I spoke Spanish for the first time: "Mejor llevanos a Lavalle entre Reconquista y San Martin - Better take us to Lavalle, between Reconquista and San Martin" which was the address for the local police precinct (are they still there, I wonder). I still remember with satisfaction the guy's reaction.
Bottom line: be cautious as you should in any place you are not familiar with but there is no need to become paranoid.