The memorial is in two parts. The stelae field above the ground and an information center detailing the personal lives of the murdered jews under ground. It is absolutely worth visiting the under ground museum. Entry is free but it is recommended to pay 4 Euros for the audio guide. There is a brief security check that causes a 10 minute queue at entry. There is a LOT of information and one can completely sink into stories of people and the way normal lives were disrupted and destroyed in a single day. The  tour can easily suck 2 or more hours of your schedule, so plan accordingly. 

 Another tip: visit the area again at night. The ghostly lights and atmosphere transport you to a different place. It is close to the Brandenburg Gate, which is also a pleasant sight at night. There are generally a lot of people around even at 11PM so it should be safe.

Another tip: during the day walk alone among the stelae from one corner to the other to experience the full power of the memorial. Whatever you do don't just look at it fromroad level