We just got back from BA last week. On our first full day in the city, we took a city tour, "Images of Buenos Aires," in order to get acquainted with the lay of the land. Both my husband & I loved it! They have a daily tour, with different pick-up sites around the city. The tour was a combination of riding a very pleasant bus and walking (none of it strenuous). Our tour guide, Julian, was wonderful. He covered not just the normal tourist sights and information, but also added hugely interesting history, geography, anthropology, etc., information. Based on what we saw on the tour, we were able to narrow down the places we wanted to return to so that we could investigate them in more depth. For example, we were able to decide that our little view of La Boca (we got out to walk on the famous little alley that is in all the guide books) was plenty and that we didn't need to waste time coming back. The time spent in the Recoleta Cemetery made us positive that we wanted to check out the other large cemetery (La Chacarita).
We thought the price very reasonable ($25US per person) and well worth it. They have other tours available, as well ("Jewish Buenos Aires" and "Paris of the South" come to mind) and I plan on doing another one when we return. Their website is in English and Spanish (http://www.eternautas.com/). I was able to book our tour before I left the States. They did ask for an American Express card to book the reservations, but they were able to deal with it when I explained that I didn't carry that card. All in all, highly enjoyable. I highly recommend this outfit.