When I first saw this name, I just thought of it as another "yuunique" spelling of a normal name, but after finding out that it's a normal linguistic variation, it's grown on me a lot.
I'm a trans guy and I chose Martyn as my name because I really like it and prefer how it looks spelt with a Y. It also sounds like it's supposed to be spelt with a Y rather than an I to me, although I'm not sure why I get this impression.