In the past, I've flown Qatar Airways, Austrian Airlines, Air Astana, and Aerolineas Argentinas, among others. Turkish Airlines lived up to my experience with those flagship carriers.
While the planes were far from the most high-tech or new, the service stood out. I was pleasantly surprised at how many times Turkish Airlines' flight attendants came around for beverage service. That kind of attentiveness makes or breaks a long-haul economy-class flight.
The complimentary tour was efficiently planned and hit most of the major stops in Istanbul. It was a bit rushed and more of a teaser than anything else. I definitely want to come back to see the inside of the Hagia Sophia and spend an unhurried day exploring the Grand Bazaar and Topkapi Palace. I suspect that's the point.
I will say that not having a hotel room to stop in was a bit of a bummer, considering my 18-hour layover. I did not account for how tired I would be at the end of the itinerary. I guess I'm not a student backpacker anymore.
All that said, it was definitely worth the $500 I saved on airfare and I would recommend it to someone looking to tack on an extra sightseeing day to their next international trip.