The HHKB's layout is a nightmare. The Fn key has to be pressed with the right pinky, and you have to use it all the time, as the arrows are not directly accessible. If you have to use the Fn layer all the time, then at least Fn key should be located in a place where it can be activated by a thumb.
And it is, you just need to play with dip switches. Then you can locate FN on the left diamond key, which just by "accident" is accessible by a thumb...
Exactly how I feel too. Love my FC660C, and will never bother with a HHKB, even if one was given to me as a gift. I'd just turn around and sell it to one of the HHKB fanboys. 
And who sounds like fanboy now?
Honestly, both HHK and FC660c are good keyboards but at the end of the day Leo just have more flaws:
No information about dip switches, not even on the box, you need to visit net to get those info. But is only 4 of them so is not too hard to learn them, but always. HHKB got this info on the back.
Switching capslock with control on Fc660c is killing red led light, which make sense. But when you look on this control key with small window for it, you just have this strange feeling...
Some key combination on Leo just don't work, famous Ctrl+Shift+Esc command for taskmanager. Also, maybe it is just me, but I had problems to get fn + arrow for PgUp/PgDn working on Mac (I'm using 4 systems atm Win/Lin/FreeBSD/Mac). Yes I know, I can use different combinations, but still.
Wobbling keys (yes, you can use spiceBar tutorial to silent/kill it) that you can't replace/customize, well not all of them...
And, then, layout. To get to this nice FN+ arrow godly combination on Leo (same as Del key), I need to move my entire arm. And as you can guess, I'm lazy and I don't like to do so...
You can like or hate layout on HHKB, is absolutely understandable. Has no metal plate, is expensive and it can feel plastic for many. But in my opinion is just better product than FC660c.