I had read about issues getting and reloading an IstanbulKart card but was preparing to give it a go once we arrived on June 11. Then at the airport I saw a big advertising poster that said all public transit could now be accessed directly with a debit or credit card.
We tried that out on arrival day and it worked great. Just hold your card over the scan plate on the turnstile, a little longer than you might think. The turnstile buzzes and turns from a green arrow to a red X. A red X confusingly means payment accepted, and you just push through the turnstile. 25 TRY / .78 USD per trip.
Worked great for me, but my brother got stymied by the red X thing and waited for it to turn back to the green arrow. Twice. Eventually we got him through. His card was charged correctly for the number of actual entries through the turnstile, not the error entries where he put down the card but didn't pass through.
Only downside we discovered was that the public toilets which take IstanbulKart as payment have not yet been updated to work directly with cards; you still needed the IstanbulKart for entry which we didn't have.
We did see one very distraught tourist who had just loaded 100 TRY onto an Istanbulkart card prior to trying to enter the turnstile. The balance was not updated and the card was not accepted. She asked us and everyone around for help, but no one could make it work. Felt sorry for her as we sailed through the turnstiles but we didn't have a solution except to pay with a card.
I am loving this new convenience, have also used it in Milan (where you also need to tap out with the card, not just in). I think it is active on Rome public transit, but can't confirm personally as I haven't done it. I've heard it works in London too. No stopping at kiosks or tobacco stores to buy tickets and then validating them.
We survived a 3-day trip in Istanbul without once getting any Turkish Liras in cash. All the bank ATMs we tried wanted an 8% surcharge and we didn't want to pay it. We did not go to exchange booths or non-bank ATMs, they were real banks, but the fees were outrageous and the same 8% at many different banks. We used credit cards, and the odd time we used cash Euros or USD (only for tips for guides and one airport transfer.)
One last caveat - I consider it rude to hand Euro or USD coins to anyone. Coins can rarely be exchanged or spent outside the country of origin, so you would have just dumped unusable metal on someone. We always asked in advance if those two currencies would be accepted, and then offered only bills, not coins. It was cheerfully accepted and everyone was able to convert on the fly and did not feel cheated.