I've known a lady ever since I was little whose name is Janis and I always thought it was a fake name. I call her son, whose name is John, Johnis because I thought it was the name Jan with "is" on the end. Recently, I found out that it's the same as the name Janice. It all makes sense now hahaha.
I'm not much of a fan of the name Janice, as I don't like many names ending in the 's' sound, and because the name has a rather harsh sound to it, but I prefer this spelling, as this makes me think of Janis Joplin rather than Janice Dickinson. Of course, the pronunciation isn't any different.