Finally got my Leopold FC660C, and I've had it for several weeks now. I like the Topre switches MUCH more than any Cherry mx switch I have tried (brown, blue, black, white, green), and almost as much as IBM buckling springs. Not only do I like the typing feel and sound, but my speed and accuracy are better on this board than any of my Cherry mx boards, including the FC660M.
The Leopold FC660C could be my favorite board if they ever issue some decent dye-sub PBT keycaps for it. In the meantime, I will probably revert to my trusty IBM SSK.