The place is convenient to rest your head for the night. It is close to the tram and Westbahnhof subway station (about a 15 min walk to the station, or you can take the tram to the station and then the train to the historic district). The apartment as such is fine to rest your head and clean. I must mention that the bathroom has two doors one leading in from the main entrance and another from the kitchen and without a lock. So you better assume that someone is in if the door is closed. I had the door opened on me a couple times, luckily I was in a decent outfit, but still not the best for privacy.
My check in story was not the best. Katrin was out of town and her room mate was not available to receive me as a result I was stuck outside the building with Katrin on the phone trying to get in touch with her room mate. After a lot of this back and forth she arranged for her friend to come over to help me and keep my bags at a neighbors place. After a long journey this is the last thing you expect and was quite annoying. However, I do realize it was a general miscommunication. She was very apologetic about it, but it did leave a bit of sour taste.
Once I got in, her room mate told me they don't have enough keys because they had double booked me. When I would have none of it, he gave me his key.
After that however, it was smooth and the rest of my stay was fine, except for the initial glitch.
I would say, if you need a place to stay and rest your head, this place is fine to do so. Just make sure you send them reminders so you don't have the experience of being stuck outside.