For a couple months now, been switching daily between two stock current-gen Mac keyboards (on my Air and the Mini at my office) and this HHKB.
For the first 2 weeks or so, maybe longer, I was struggling to adjust to the location of the Delete key in particular, and also getting used to the arrows. But mind you, I wasn't using the HHKB anywhere near as often as my work keyboard, so I bet the adjustment would have gone faster if I was going home and typing on this for hours at a time.
Anyway, at this point, I can switch freely without any issues. And I'm really starting to pick up the HHKB FN arrow keys as well -- definitely not looking down or hesitating at all at this point. You don't use CTRL as much on a Mac anyway, so that isn't as much of an adjustment as I bet it would be for a PC user.
The key feel of the Mac keyboards is of course a huge disappointment every time I go back, but not much to be done about that I'm afraid, without blowing another $250 to put one of these on my work desk.