Susan's flat is a fantastic place to stay in the Mitte district of Berlin. You are spoilt for choice regarding tourist attractions that are within easy walking distance (15 - 30 minutes) and it is well served by public transport (Ubahn and trams) to reach those further away. The local bakery, which is very close, is a real treat with an amazing selection of breads, buns, etc cooked on the premises. There are a variety of local restaurants, all of which seem very popular with the locals. Hopefully in future return visit to Berlin we will be able to stay there again.
For anyone arriving by air, the S9 service from the airport to Hackescher Markt is a better choice than the S45 service - the latter required at least two changes on the Ubahn. The Markt is a 10 - 15 minute walk from the flat.