Just flew with them 2 weeks ago. I have taken a couple of flights so far. The inflight service is quite good and they have better meals compared with some Asian carriers. Their lounge in Istanbul is very nice serving various Turkish food and drinks with decent Wifi. I really like the lounge.
Some negative points: They often reschedule timing. If you have connecting flights, you need to make sure you have enough time for transfer or else, you need to rebook flights. As others mentioned, Istanbul airport is gigantic. There is the inflight video clip on board to guide you how to reach departure area after your arrival in Istanbul.
The ticket cost is generally cheaper than European carriers when I take them from where I live.