Yeah, if I ever wanted to use Gwenhwyfar, I'd go with the anglicised
Guinevere (unless, of course, some awesome chance presented itself that would allow me to move to Wales, then I would totally use Gwenhwyfar).
Gwenhwyfar is so romantic-looking and its meaning is relatively mysterious... but also unpronounceable by 98% of the population here in the States. :( I could never use Gwenhwyfar for the same reason I could never use another Welsh favourite,
Myfanwy. *sniff*