For starters, let me just say Schønefeld is a very simple and I'm guessing young airport, because it's facilities are really not top notch and it's a petty damn long walk away from connecting infrastructure. That being said, for us, young people, travelling in the summer, the walk was not such a problem, but when it's cold or if you're not that into walking, you are pretty much forced into grabbing a cab -which is quite expensive in Germany.
That being said, once you're aboard the train you're on a part of a great interconnected web of transportation. Trains, trams, subways and buses, Berlin has a lot of them all. Figuring them out is another matter, but overall it's a decent system. This too, is pretty expensive, which is pretty well illustrated by the fact that in the course of the weekend I was accosted by a total of five beggars -three of which where asking for tickets with hours left on them rather than money. Figuring the transportation system is paramount -Berlin is a huge city spread out wide, and even though we walked our shoes off most days, we'd have no chance to see all that we wanted to see without the weekend transportation pass.