Different beasts entirely in terms of looks, size, weight, switch options, and (probably) customization. I don't own a RW keyboard, but I've been very tempted by a few of them. I suspect that they're probably worth the investment if you want to build your own and like their designs. The big wins with it are color choices, style, extra PCBs available, dampening and weights.
I've owned the HHKB. With it, you're paying a reasonable amount for a plastic case, great keycaps, great switches and a very specific layout.
I'd be interested in hearing what owners think of their Koyus and M60-A SEQ2s once they get them and have used them for a few weeks.