A great city- loads of different areas to explore- Palermo, Recoleta, San Telmo, La Boca are definite must sees. It's worth staying near a metro to help you get around easily (the red line was the most useful for us).
Insider Tip:The Tango Complejo package is a good place to start (tango lesson, dinner and show- highly touristy but worth it for the introduction). Stay in Palermo Try Persicco's ice cream. Eat at Cabana Las Lilas in Puerto Madryn (expénsive but worth it)