Overall we enjoyed our stay. We are a young couple on their first time in Turkey. Mustafa's place turned out to be ideal as it is located at walking distance from all the must-see monuments of Istanbul (I can't emphasise more how this is important as public transport is a nightmare in the area). Moreover, the room is quite new and the bathroom is large.
On the downside, the neighbourhood is very dirty and it almost looks like a dump in some streets. During week days you won't have much time to relax as there's shoemakers working constantly until late (you can hear noise coming from the streets until 11pm). Also, the windows and the balcony of the room face a shoe laboratory (which is less than 5m away) so we had to stay with the curtains closed throughout our stay to have some privacy. On Sundays the area changes completely as there are no workers and it's quite enjoyable walking around.
I definitely reccomend Mustafa's place if you're young, on budget and not fussy. It's probably the accomodation with the best price-quality ratio in Istanbul. However, if you would like to have a bit more of quietness during your stay in an already chaotic city, then I suggest to spend a bit more and look for in another area.