The layout on a Mac
I used the original wired model of the HHKB (the real one with Topres, not the "Lite") for several years on a Mac, for programming and general use, and my experience might be useful to those wondering how practical the layout actually is.
The first thing to say is that having the correct OPTION and COMMAND keys in the correct places on both sides of the spacebar is important on a Mac if you touch type. In macOS OPTION is a secondary shift to get at extra characters like the curly quotes and en dash. So many keyboards are designed mainly for Windows and provide only CTRL on both sides. This is the only 60% keyboard I know of that actually gets this right for macOS users.
I had no problem adapting to the position of CTRL, having used the DEC VT220, but as a heavy emacs user in this period I found it a bit of a finger twister, as the Space Cadet had CTRL where it is on a normal keyboard and is what emacs was designed for (as far as emacs can be said to have been designed). macOS also uses the emacs shortcuts for line editing on the command line and in GUI text boxes (go on, try it out!) so that's handy: CTRL-a to go to the start of the line, CTRL-k to kill the rest of the line etc.
The arrow keys, PAGE UP, and PAGE DOWN are there but on a separate layer which you have to chord with FN. This gets tiring. Forward delete (DEL) is similarly available as FN-BACKSPACE but as mentioned for anything line-oriented the emacs CTRL-D works on macOS. HOME and END are little-used on macOS because they only scroll. What you want is COMMAND-UP and COMMAND-DOWN, which are dire on this keyboard.
I eventually reached the point where I was worried about injuries from all the emacs chording and the chording otherwise needed for working in the GUI, prompting a rethink of how I should be using a keyboard. My decision was to abandon emacs, use the mouse more, learn to touch type, and use CAPS LOCK properly instead of stretching to hold SHIFT down the way a lot of programmers do. As I now needed CAPS LOCK where finger L5 could reach it easily this meant the HHKB had to go. I use a US layout tenkeyless with Cherry browns now and edit mostly in Xcode with occasional detours into vi.
I think I used the keyboard for about 3 years, so this wasn't a quick tryout.
I do think had I been a vi user things might have been different as the arrow keys aren't used as much as emacs (where I found CTRL-p etc. just too unergonomic on any keyboard.) If you sit in vi in Terminals all day, this keyboard is definitely worth looking at due to the Topre switches and compactness.