Using it for my favorite character in the book I'm writing. I figured that I could use the Americanized pronunciation to make a more accurate impression on readers. It also helps me convey the eccentric naming patterns of my world, and of Siv's hometown.
Simple yet comely, would be a great middle name as opposed to the overused Lee or Marie. It can work out as a first name too, with a more developed middle name. Anyways, I like it.
When I first read the Myths of the Norse, I came across this name, but spelled "Syf", and even though it may not be correct, I must say I still prefer it that way. It seems a little prettier and more feminine, more Goddess-like. :)