I've sung the praises of the apartment we stayed at in Buenos Aires before, but they've recently started up a website for their rentals so I thought I'd give them another shout out. They are now located at http://santelmoloft.com.
Staying at the loft was a different experience than any other vacation rental or hotel I've ever stayed at. I think there is always a concern when you stay at a rental on vacation that you might be sacrificing quality or the helpful advice of hotel staff. Not so with San Telmo Loft. Best concierge service ever! (Although it feels funny to call what they provided a "service," as I now think of Angela as a friend). While the plane fare makes it hard to get back down to South America, it will happen sometime soon enough, and when it does we will absolutely stay with them again!