Flat's location is great. There is absolutely no doubt about that. You are in the center of all Thessaloniki's vibrant life. Flat itself is just ok, I would lie if I would say that I have not seen better options in the wider area, in more or less the same price range.
In my opinion if you are not bothered with a low ceiling flat or bad wifi/mobile reception or in general you are not spending much time in the place apart from a sleep and just exploring the city it's an ok option.