As many former guests already pointed out, Ali's place and the whole handling (check-in, tips for transport etc.) was fantastic!
The flat is small but definitely big enough for 2 people (I stayed there on my own) and has everything you need: a dangerously comfortable bed (I slept VERY well there), a nice clean bathroom, good water pressure and hot water in the shower, good heating, a very clean and big fridge, a closet with hangers, a water cooker for a quick tea or coffee, enough(!) towels, enough outlets (both at the bed and at the table/workspace and a very nice terrace which I didn't use due to the cold weather in December. In summer it must definitely be very nice to sit there with a glass of wine or a beer!
The place is perfectly located within the nice and safe neighborhood of Beyoglu in a very quiet street which is nonetheless super close to the next metro station (Sishane) and the tram station (Tünel) which leaves you right next to the Galata bridge and provides you with transport to Sultanahmet, the piers and all the major sights. The neighborhood has many shops for musical instruments, souvenirs and good food as well...
Ali was a very nice host and the communication could not have been any better. Just ask him anything, he will deliver ;)
Self check-in is possible and worked fine!
For transport from the IST airport get a Istanbulcard, take the Havaist bus (18 lira) to Taksim square from where you can either walk or take the metro to Sishane station.
My favorite tip: take a ferry to Kadiköy (only 3 lira) just before sunset and enjoy the ride to the asian side and then enjoy some of the good food they offer there, plus there will not be many fellow tourists...
So, 5/5 would book again, I'll not waste time looking for anything else the next time I'm around!
János