Frank's apartment was absolutely perfect for mine and my girlfriend's stay. Located in a great place, with easy access to the train station (Südkreuz) which was a 10 minute walk away, and with trains into central Berlin going frequently. It was in a friendly neighbourhood, and Frank has brilliant taste. The kitchen was great - albeit the hob a bit confusing to use - and the bathroom also aesthetically brilliant. Frank was lovely too, and easy to communicate with, replying back to messages very quickly. I would definitely come back here if I came to Berlin again, as the apartment was functional, stylish and perfect for stay. We even managed to make the speaker work, which was perfect to have some music playing when cooking.
Only pointers would be that there is no freezer in the apartment, which, in the Berlin heat was quite arduous as we would've loved some ice for drinks, however not a necessity as the fridge solved this to some extent. The shower pressure was also quite tedious, with the temperature having to be continually changed. However, these were minor flaws to our whole stay, and did not dampen the experience in any way. If you're looking to stay in Berlin, you really should stay here. Thanks Frank!