Erhan's apartment is located in a residential area very close to Istiklal Street - where you'll find amazing restaurants, bars, and sights. It's conveniently located to those attractions but it is located on the 4th floor of a somewhat conservative residential walkup building, so keep that in mind if you have issues with stairs (in addition to Istanbul itself being built on hills).
Erhan is very helpful and gave us amazing tips for restaurants - which even some of my local friends hadn't heard of. If you go by his recommendations, you're sure to have an amazing time. He was also quick to respond and helpful with any questions we had.
One thing we did have to deal with during our stay is the smell of sewage in the bathroom. This is an older building and had some type of drainage pipe that would smell every once in a while, and we'd have to pour hot water and bleach down it to remove the stench. Erhan was helpful in telling us what to do and even did it for us once when he came by, but it did feel like a chore that we had to do while on vacation.