My family lived in the Arenales section of BA for a few years. I still resided in the states at that time but I made a couple of extended visits. It's a beautiful city and I've managed to keep some very close friends from those trips.
It was pre-cycling in my life but I can tell you that riding down there would take guts. The cars don't follow any kind of vehicular laws. That aside, I think it would be a great place for a ride but I don't know any specific routes.
Keep close eye on your wallet too...