My experience of investing in Istanbul has been very positive and better than any other European property market. Since 2007 I've owned property there and even during the height of the crisis prices have remained stable. My units have been almost constantly rented and I am seeing 10% net yields.
My feeling is that this market has a lot more growth left in it due to the demographics and the success of the economy which grew 11% in 2010 and is forecast to be one of the fastest growing in Europe. Then look at the rest of Europe. Don't confuse Istanbul with the rest of the Turkish market, the dynamics are very different.