I used to like this name - particularly the way this spelling looked - but eventually I grew to dislike that all the vowel sounds were short-Is (basically schwas). I think Sibylla is a slight improvement, but someone with that name will probably get called Sabella, or even Isabella, a lot.
I don't know why the Sybil spelling became more popular, since to me it looks like SIE-bil (or "sigh bell"), and I actually like that less than Sibyl.
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