Actually, I answered the wrong message before....
Thanks for the report on Kasibeyaz in Florya. It's a bit out of the way for most visitors, but probably fairly easily reachable by banliyö train from Sirkeci Station, eh?
Starbucks serves coffee. In the USA they're famous for complicated coffee-based beverages with elaborate nomenclature (a typically American thing).
By contrast, I'm a purist. I like things simple, straightforward and high quality. Starbucks is the sort of place where I'd expect the actual coffee to be low quality because it's only the base for drinks into which other flavors and preparations are added. But--surprisingly--Starbucks' coffee (plain) is very good. It's a dark roast, brewed fairly strong (unusual for American coffee, which is usually medium brown roast, brewed weak).
I'm just about the only person who walks into a Starbucks and orders a small, plain cup of coffee, no milk, no sugar, no nothing. Just coffee.
I'm glad Starbucks is in Turkey because my experiences with coffee in Turkey are usually disappointing (with the exception of Turkish coffee, which I like).
In my experience, if you order anything except Turkish coffee in Turkey, you get bad coffee, even in expensive hotels. The beans are often burnt (way too dark), and I have the feeling that too often it includes some cavcav (cavcav is coffee brewed from previously-used grounds, so what you get is weak, bitter, dead-tasting).
As each cup of Starbucks coffee is made fresh, this shouldn't be a problem.