I’ve been to Turkey recently and used my Monzo card almost for everything. The experience was great! Here are a few tips & thoughts that I had:
Basically the card was accepted everywhere (using EMV) and I haven’t had any issues with it.
I was able to withdraw cash from ATMs without any fees. I’ve used Garanti Bank’s atms and it was straightforward. Only one thing to be careful about is at the last step the atm will ask you whether you want to be debited in GBP (with 4.5% exchange fee) or TRY. Always choose TRY.
This is expected, but merchant names are pretty useless. I’ve never got an accurate merchant name/address except once (for unsurprisingly Simit Sarayı.) As a tip try to always suggest the right merchant as soon as possible so you don’t forget who it was. Everything should be there on Foursquare.
Monzo’s exchange rate was quite good, although at one point I was able to get a better exchange rate from a local exchange office (that I personally know.) It’s definitely better than anything you can get from most offices/banks.