So this question probably has little to do with experiences and more with personal opinion.
I am going to be in Athens and Istanbul from 11/21 until 11/30. To get from Athens to Istanbul I was going to fly, as it is the quickiest and eaisiest. The cheapest airfare I've found is through Aegean Airlines, and is $114 US one way. The problem is, that flight is at 1:00PM and get into Istanbul at about 2:30PM. By the time I'm into the Old Town and at my hostel, most of the day is shot.
Conversely, Olympic Airlines has a flight at 7:45AM and I'm in Istanbul by 9:00AM. This means that day is not a waste, and is the best possible scenario. The problem is, the Olympic flight is $250 US one way. That's about a $135 US difference.
So, as fellow travelers, what is your opinion. Would you personally spend the extra $135 for a full extra day in Istanbul. If you do the cheaper flight, you have to be at the Athens airport by 9:00AM, and aren't into your hostel in istanbul until probably 4:00PM. The day is mostly shot. Is the extra money worth being in Istanbul bright and early at 9:00AM for a full day?