We went to Berlin just for three days with my wife, although it was literally freezing, we had an amazing time.
Jon's apartment was located in the very center of Berlin. It is close to all major locations. You can walk anywhere but if you're not a fan of walking, than there is the subway just around the corner. There are also a lot restaurants, bars etc. very close to the apartment.
The apartment is really cozy, there is a lovely terrace, which we enjoyed even during winter, it is very nicely decorated I imagine, it would be a great place to chill in spring and summer. The bathroom, kitchen are all in great condition.
Jon was not there when we arrived but a friend of his helped us get the key. We followed Jon's instructions and had no problems at all.
Overall, it was an excellent experience for us, I would definitely recommend staying in Jon's apartment.