I agree with the previous comments about build quality. The HHKB is built differently from Realforce boards such as the RF87U, but I do not consider the build quality of the HHKB to be inferior to that of the RF. I have owned both boards for some time, but the one I use most often is the HHKB.
Although my RF87U feels more solid than my HHKB, I find typing on the HHKB less fatiguing and more fun. The HHKB is more resilient, and its construction makes the "thock" of its keystrokes more pronounced than on the RF.
Moreover, as others have said, the HHKB represents a beautiful synthesis of components yielding an overall typing experience that is more than the sum of its parts. It has all the ingredients I would want in a keyboard: compact 60% form factor, efficient layout, dye-sub PBT keycaps, excellent switches, and attractive design and styling.
Comparisons between the RF87U and HHKB often invoke a car analogy -- typing on the RF is like driving a luxury sedan, whereas typing on the HHKB is like driving a sports car. This does not mean that the quality differs between the two boards. As for me, after a while, the luxury sedan becomes boring. I prefer the zippy performance of the sports car.