Hm... I do agree with all the praising comments about onboard service and amenities. Also the lounge in IST is great.

Just to add a couple other things:

1) The boarding procedure is implemented in a disastrous manner, particularly with long-haul flights, where you think you're attending a bazar. This affects IST (where gates are too small sometimes and not conceived for such big flights), but also other airports (I have had awful experiences at IAH). I wonder why they don't copy from UA, who do it so much better consistently. This problem delays flights systematically and is to a large part due to the staff's lack of competence and care.

2) Check-in: Very long lines, even for Premium check-in, in IST as well as elsewhere (IAH check-in for Y can take forever). Security: if departing from IST it's TERRIBLE, endlessly long lines, again even for Premium passengers... The airport can't keep up with TK's massive growth, so it can be quite a mess. Two hours can be just enough time to comply with all requirements if the day and the time is busy... So beware!!!!

2) TRANSFERS at IST.... Oh my God: be sure to take the right passport line, make sure that you have your e-visa if needed (these two if you're flying domestic after your long-haul). Be aware that you have to help yourself instead of expecting someone else's help: ground staff at IST is often not proficient in English and if they are to some extent often it only shows how rude or at least harsh they can be... Angrily pointing at your rights as a customer can be by far less efficient than trying to become a friend of the person who's meant to help you

My experience: as a general rule, operations on ground can be DISASTROUS without any exceptional circumstance. One of the things that will make TK stay a 4-star carrier for a long time still is definitely that: they are very far behind other European or even US carriers in their ground and hub-management, including their staff's manners and attitudes with passengers. Still too Mediterranean. Yet, once you make it on-board it's gonna be a dream :-)