The apartment is wonderful. Although it's a bit off the beaten track of Central Berlin but transportation is so great in Berlin that it doesn't really matter.
I stayed here for 2 whole weeks and I enjoyed the apratment very much. The apartment is kozzy and has everything you need - very clean, kitchen with all the utensils you need for a short or even longer stay (I cooked for myself all the time and found everything to be there in their kitchen).
Washing Machine and Dryer are also available in the next building and Heike was very pleasant in guiding me through it all.
Heinrich and Heike are great hosts. They replied to every question I had and at a reasonable speed. When I had a slight problem with the shower although they were on holiday out of Berlin they took care of it the next day and that's all you actually expect of you hosts when not in you're own country :)
All in all I had a great time at this aprtment and I strongly recommend it.