Our stay in Konstantinos' apartment was great. The location is superb - directly in the heart of the city. You just have to step out of the door and you'll find nice shops, restaurant and bars within a minute's walk. The flat itself was very clean and comfy. We definitively would stay here again!
One thing to be aware of if you visit Thessaloniki by car is the rather crazy parking situation. It was really hard to find a parking space so we had to circle around for quite some time.