I used a three size corner cutter that I got on Amazon for about $15. It's called the "Sun-Star Kadomaru pro corner cutter".
I'm part way through the Noblet using the same 3mm corner rounder, and I already love the way I can get my hands all over these cards now without sharp pokes. And the change in appearance is minimal (and to me, even preferable).
Thanks for your post & photos! They helped me choose a corner rounder and gave me encouragement to go for it. (Plus it didn't hurt to see Florney's
note that he'd planned 3mm corners anyway, but the printer couldn't accommodate.)
EDIT: Surprisingly, it took me about 10 seconds per card, so under 15 minutes for the entire deck, rounding 1 at a time. Nothing snagged or came out unevenly. They look like BSwett's pics, but here's a different angle.