If I say one more nice thing about Turkish Air, people will think that I am on their payroll. But when I keep reading about all of the people being inconvenienced by the canceled flights of some of the other carriers, its hard not to be appreciative of Turkish Air.
In the past 24 months I have been on 14 Turkish Air flights. Later this week I start a trip that will add 4 more Turkish Air flights to that list.
None have been rescheduled or canceled. Nor have I heard of anyone on a canceled or rescheduled Turkish Air flight (but yes I am sure it happens)
None have been late arriving, that I can remember (which means if they were a little late, it had no impact on connections, etc.)
On two occasions I have had change my itinerary; both times done on the phone, both times took about 30 minutes to complete and one time cost $112, the second $27.
Yes, you have to change in Istanbul no matter where you are going. It's a well run airport for that, better than Amsterdam, way better than Paris or London.
Yes, if you are traveling from a city that Turkish Air doesn't serve (they code share with United), and if you are going to someplace in Northern Europe, it can be a long trip.
Pay particular attention to their Multi-City Fares. They can be as cheap or cheaper than round trip fares. Prices are not the cheapest, but far from the most expensive.