Is this a common turkish phrase? I've never found myself to say "fortunately you exist" in english :P
The exact translation looks weird, I admit, but maybe if I translate the meaning, you'd get the phrase. It's something like "I'm glad I have you in my life". So when you feel that way, you can use it :D
Yes it is very common, it can be used as a thank you perhaps in some contexts and we also tend to use it while celebrating someone's birthday :)
I said "neyse ki" and it was accepted.
Actually that is not false. But we use "neyse ki" in bad situation and as a saying for consolation. For example: "Çocuk ağaçtan düştü. Neyse ki bir yerini kırmadı."
This brought "İyi ki varsin Eren" to mind :-(
Why not iyi varsın?
This is just a saying. "fortunately"="iyi ki"