Many people travel with luggage on the bus, though the bus can get totally packed and the scene at TXL to catch the bus can be a bit of a zoo. With one suitcase you should be fine. Note that Alexanderplatz is the bus terminus, so when you return to the airport you'll be first to board But I noticed that at some times of day when the traffic got heavy they would turn the buses early, around Brandenburg Gate, in order to keep on schedule. In that case you need to take the local bus from Alexanderplatz over there to catch the TXL. This doesn't happen often but it's worth noting; there is a lot of construction on that part of the route and it sometimes causes delays.

There is a ticket machine outside at TXL where you catch the bus. It's a fine way to get into town, in my experience.