Overall we had a great stay at Erol's place.
It takes about 10 mins or so to walk to the closest metro station (Şişli). The apartment's location is excellent if you don't mind that walk. You can hear traffic noise from the outside when you open the windows in the apartment, but it's quiet enough with the windows closed, especially in the evening.
Cooking utensils were minimum, but we mostly ate outside and didn't really cook. So it wasn't a problem for us.
Once the boiler stopped functioning and there was no hot water, but Erol came to rescue immediately after I messaged him.
The internet connection was not the fastest and I assume it's like that with most other apartments you might consider. So, again, unless you really need a fast internet connection for your work or something, it's fine.
When you consider how much you'd pay for staying at this apartment, I believe what you get is very, very reasonable. We would happily stay at Erol's place again if we visited Istanbul in future :-)