We stayed at the St Oberholz for 2 nights over a weekend and it comes highly recommended. The location in Mitte is fantastic (the White Trash pub is within easy walking distance), the apartment is incredibly stylish and the staff extremely helpful. Emails were answered straight away and all staff, like most Germans, speak excellent English. The café downstairs (part of the same set-up) is also great, with excellent food and coffee. Both, the apartment and café, also have free Wi-Fi which was very helpful. The apartment also has a kitchen, equipped with coffee and tea.
Note that the building doesn’t have a lift. Not a problem for most people but might be important to some.
I would definitely book the apartment again on our next trip and have recommended it to all my friends.More