I had a chance to try the HHKB recently and its a great board, it was a display piece so I dont have a full experience of it but it sure felt great to type on. From what Ive seen and read, people prefer the HHKB layout compared to the standard ones. Noticeably, the backspace and ctrl location on the board. Also the fn layer seems very intuitive, but as with any new board that may take time to get used to. The Realforce, HHKB, 660c all has its merits and all have a unique/different typing experience thats hard to express. When I was in the market for a topre board, I would have went with the HHKB if it wasnt so hard/pricey to get one here. Now Im thankful it was hard to get since Im in love with my 660c.
PS. I would stay away from Novatouch... seems like theres more people selling theirs than keeping it in the Korean community. (buying it as a donor board is a different story...)