Architecturally magnificent structure! there is tight security and usually crowded line, they may push and squeeze too many people in elevators! but overall its a nice experience and beautiful view...
We reserved a table at the restaurant on top, and was able to skip the line. The restaurant was not to bad, and not expensive if you consider the location! Really recommend to go see the dome.
Even if you did not reserve online in advance you can still get in - simply call the Kafer cafe located by the dome and order a table for coffee and cakes . They have delicious desserts.
A must see in Berlin. Reach via bus 100 from Alexanderplatz. Do book before going. Request goes to a govt. agency and you need prior permission. Free of cost. Some of the amazing views from the dome.