Schönefeld requires an extra cost ticket, the Tegel bus works on your normal AB ticket (day pass or single ride). Having watched the addition of rail service to OAK (and SFO), I don't really see the attraction. In the case of Oakland, the bus was faster, half the cost, and dropped you off closer to the airport. With Tegel, the bus is relatively quick (around 20 minutes from the Hauptbahnhof) — my only complaint is the lack of ventilation and cooling onboard (which is often a problem on trains too).
Quite frankly New York has a similar problem. There is rail service to JFK but requires an extra cost ticket and it's a fairly long trip (moreso if you're taking the subway instead of LIRR). I'd be willing to take a bus if it got me there quicker for less money. Yeah, Newark has rail (but it doesn't run particularly late).