I stayed at Sonja's for 6 weeks. The apartment was clean and comfy. It had basic cooking equipment, a small plug-in stove and a fridge. I would totally recommend this place for short stays but probably not for long periods of time - cooking and the outside-shower are not as comfortable when you stay for more than a few weeks.
In my case, I was here for studying purposes and the small table/desk provided proved to be perfect for this. I stayed over the summer and even though the room did not have A/C and we had some pretty warm days, I was able to sleep comfortably with the window open - if you are a light sleeper though, the noise from outside may bother you.
The location was perfect, 15-20mins away from the city center (Hauptwache) and with plenty of places to eat nearby as well as a supermarket (Rewe), convenience store (dm) and 2 Ubahn stations close by. There is also a nice park 3 mins away if you want to take a stroll or go for a jog and it is 1min away from the Goethe university.
I never saw Sonja but the check-in and out process were very straightforward. I contacted her beforehand and she left the keys in a small box outside and provided a return box for the checkout process. Anytime I had a question or concern, Sonja was very quick to respond (usually within 10 mins if not under an hour) - which made it very convenient.
In general, would recommend for short stays without a doubt!