The banks at the airport charge commission to change money; in the city there are plenty of private exchange shops as well as banks (banks waste time). They do not charge a commission in the city (except some exchange vendors at the grand bazaar).
Using ATM for cash works fine everywhere. US$ and Euros are welcomed by almost any vendor, except museums. Credit cards are also good almost anywhere, though at times you are quoted a more competitive rate for cash.
Enjoy your visit!
Oguz Kosebalaban
Your host in Istanbul
This tip is provided by supplier M05462