The apartment was amazing: large (really large), clean, quiet (we've literally never ever heard the neighbors, though you can hear the street a little bit) and very well-equipped. There is a huge collection of CDs and a stereo system, you can listen to anything there is. The apartment has it's own WiFi router and the speed is very good (both for work and for watching movies online). For those like us, who got tired of small studios and wanted to live in a comfortable apartment for a change, it was a great place.
The location is very good and easy to find, and also it's easy to reach by taxi. The district (Şişli) is pretty quiet, almost the opposite of Beyoğlu (we lived in both and both are great, it all depends on what you're looking for). There are stores and shops and cafes in the neighborhood, and an amazing bookstore, if you're into books.
The communication with Murat has been very clear and polite, he has always responded fast and offered his help with whatever we needed. And the check-out has been very simple, you don't even need to wait for the owner to pass him the keys, you just leave them in the apartment and that's it. So we would definitely recommend this apartment to whoever's looking, and thanks again for a great stay :)