I stayed at Betal's apartment in Beyoğlu for two weeks, and the experience could not have been better. The flat is in a fantastic location in a trendy neighbourhood with loads of cafés and shops within five minutes' walk. Despite being in a very busy spot, the flat is a cosy oasis away from the bustling streets below. The bed is colossal and I slept like a log for the entire trip.
Betal was the perfect host, keeping in touch regularly to make sure everything was OK, and offering recommendations for local places to eat & drink. The only thing I would warn people about is to be prepared for a lot of traffic noise, day and night; Beyoğlu is a busy district. That said, the bedroom is at the back of the building and insulated from most of the noise from the street, so it's not really a problem.
All in all, this is a perfect base for exploring Istanbul, and I would highly recommend it.