Serdar's place is fantastic. As another reviewer said, the best part of his place is the location. Kadikoy is the best place to stay in Istanbul-- it is a fascinating neighborhood full of great cafes, nightlife, an awesome produce market, etc., but it also has some authenticity, and it's not a tourist bubble. Serdar's place is on a little one-block street in Kadikoy that is surrounded by some of the most fun and interesting places-- but the street itself is also more quiet than the surrounding neighborhood. As a result, we were never bothered by noise, and we could sit at a nice cafe for a cup of tea in the morning before heading out into the bustle--or grab a beer at a next-door, laid-back bar with a garden before turning in for the night. Or, alternatively, we could walk a block or two, and be right in the middle of the Kadikoy nightlife or produce market. Also the ferry from Kadikoy to the European side is easy, cheap, and fun. Best part of the trip, actually.
The flat itself was nice, and very clean. It's a little spare--no big-screen TV or hot tub--but very comfortable. The pictures on the website are a good representation. I was worried based on the pictures that the bed might be too small for two people, but it was fine.
The host was very responsive, friendly and helpful throughout our trip. He responded to our first email quickly, was on-hand for our arrival, wrote us a list of suggested places to see, etc. etc..
As far as drawbacks are concerned, there really aren't very many. There was a very minor plumbing issue (a little leak in the shower lining) but it never caused us any inconvenience.
Finding the apartment on our first night was a bit tricky--don't expect the cab drivers at Ataturk Airport to be able to find the exact address. But it was only a problem for us because I had foolishly let my phone battery die during the flight. Serdar was sitting in the flat by his phone when we arrived (at midnight), waiting for my call/text/email/whatsapp, and ready to help us, so if you have a phone or email device handy when you arrive you'll be totally fine. Just take a cab to somewhere central in Kadikoy (maybe the ferry dock) and you'll be a couple blocks from the flat.
You will hear the muezzin in the morning, but that's pretty much unavoidable in Istanbul, and the muezzin is one of the best we heard on our trip (it matters). It's not awfully loud, but you'll hear it.
I'm trying to be as nit-picky as possible because I find that kind of review to be more helpful, but really it was a perfect place. My wife and I said out loud at least five times how happy we were with our choice. Perfect location, clean comfortable flat, great host.