The parish where I work is singing the Creed in English at Sunday Masses during the Year of Faith. I spent part of the day today rehearsing it with students in the Religious Education program, who were remarkably good sports about it.
The ICEL Creed, which we are using, is based on Credo I, rather than on the more widely sung Credo III. For my particular purposes, this is a huge help. Credo I is much more repetitive than Credo III, so introducing a brief introductory phrase is like introducing the chant as a whole.
And of course, Credo I is much older. It has an older, more foreign sound than Credo III. Singing Credo III, one hears a chord progression in the background, and not just from having heard the organ accompanying, but because the music seems to have been written with chords in mind.