@strider8: Yes, I have had both. I sold the FC660C, kept the HHKB, and now I have 3 HHKBs -- two for my two workstations at the office and one for my home office. The HHKB is my favorite keyboard.
The FC660C is also an excellent board. It uses plate-mounted Topre switches, which make if feel more solid than the HHKB, which has case-mounted Topre switches. However, I found the plate-mounted Topre switches to be harder on the fingers and more fatiguing for prolonged typing than the more yielding case-mounted switches in the HHKB.
The HHKB has all the features I have come to appreciate in a keyboard. These include a 60% form factor, Topre switches, beautiful dye-sub PBT keycaps, overall appealing style and appearance, and, most of all, the great HHKB layout.
People kept recommending the HHKB to me, but I resisted for a long time, because I thought the layout looked too weird. After finally trying it, I was hooked almost immediately. The location of the left Ctrl and Backspace seemed perfect right away. It took a little longer to become accustomed to the cursor diamond, but now I wouldn't have it any other way. At first, I used remapping software to change the location of the arrow keys, but soon I came back to the default HHKB layout.
I have a large number of keyboards in my collection, and whenever possible, I remap them to the HHKB layout.
Of course, everyone is different. In the end, you will need to try out the keyboards that interest you and select the one that suits you best.