I stayed at Erdem's with my partner and 3 friends and we were really happy with the experience. The arrival was easy as well as the check-in and check-out. The apartment was very clean, the terrace has great views and in general we had everything needed for a short stay. Something that is important to know is that there are two bathrooms but only one of them has a toilet.
About the service, it was surprisingly good: one night, around 4am, the boiler that is inside of one bedroom started making a lot of noise (it had been noisy before, but during the night it had always been quiet). I contacted Erdem and in less than 5 minutes I had an answer and even though the problem wasn't solved at that moment (he offered to get someone to come, but I decided not bother anyone that late), it was all settled the next day. He was always reachable, friendly and helpful, as well as his assistant, Merve.
We also payed for their transfer to the airport the last day since we had to be there very early and the trip was very pleasant (it took us half the time than it would've in public transport and the price was reasonable, so it's very recommendable).
My friends and I felt in love with Istanbul and the stay at Erdem's was definitely a plus to the whole experience.