If you have all the time in the world, try the Colectivo, but this is very confusing, the Guia T guide is not the easiest thing the world to comprehend, and you must have coins to pay for the ride. You need to know a bit of Spanish, at least to ask if this goes to the place you want - because many streets are one way, it is not easy to figure out which direction a bus is going, so ask the people waiting in line, or ask the driver - va a Sante Fe for example. They might reply otro meaning take the other bus on the same route - even though the bus may have the same number, many times they run a shorter or slightly different route. Or he may say no, wrong direction, then ask or look for the next one way street for the bus in the opposite direction. So, this is not a easy task, the guide is not very good in that it does not show the path and direction or bus-stop for each bus, so this option may not work for every tourist.
An excellent internet guide which also provides information on how to get from point A to point B and colectivo number and map is Viajo Asi web site. It offers maps, walking directions, subte directions, as well as colectivo directions. The directions from that website were usually correct, though in a few cases it reported an incorrect starting point. Therefore, always confirm their directions with the route description in the Guia-T.
The colectivo is a good way to take it slow, see different parts of Buenos Aires. The key thing to note is that since most of the streets in the city of Buenos Aires are one way only, the ida will be on different, adjacent street from the regreso route.
To pay for the ride, most tourists can just say noventa, por favor, to pay AR$0.90 (for short distances usually under 6 km or less), or AR$1.0 (for 6-12 km, covers all of Capital Buenos Aires area) [prices as of March 2008], when you are in the bus and in front of the automated pay station that is behind the driver. Enter any combination of coins, change will also be available, take the printed ticket, and move into the bus. No paper bills accepted. The current prices as well as details on how to use the automated collection box is available at Xcolectivo.com.ar Tarifas Spanish web site.