Highly recommend Yusuf's place. First, Yusuf is very welcoming and eager to please.
Second, you simply can't beat this location. Literally outside of your door you have the entrance to the tunnel where you can take the funicular up the hill to Istiklal Caddesi and Galata tower, Across the street you have the Karakoy metro stop which you will take you directly to Sultanahmet station (4 stops). You can take the ferry to Kadikoy on the Asian side and visit Kadikoy as you are just around the corner from waterfront. There is a bike station right outside the building too so you can grab a bike and ride over the bridge and along the Bosphorus. Also, you are in Karakoy an amazingly cool neighborhood. The atmosphere both night and day is really fun and interesting. You can't go wrong.
The apartment itself is a very very comfortable penthouse studio and is a great place to relax between your adventures. It is a great value too. Just be aware that this is an old building and there is no elevator so you will have to walk a few flights of stairs. That was not a problem for me at all. Plus it keeps the rear-end in shape. ; )