(Full disclosure: I have a Happy Hacking Pro 2. I also have multiple Filco Cherry MX keyboards).
The difference with HHKB is:
1) the Topre switches over standard Cherry switches; it's personal preference, but they are more expensive. In my opinion they feel a lot nicer for long sessions. Very satisfying to type on.
2) It's made in Japan — I believe Topre Realforce are too, but they're also expensive.
3) The layout/size is (fairly) unique, and very awesome. Not having to move from the home row to hit any key at all feels great.
Anyway when you amortize the cost over how long these keyboards (any of them) last, and how much you use them, the price difference isn't such a big deal.
Leopold is a great brand, and they were my top pick till they ran out of stock. Instead I bought a DAS with brown switches and haven't looked back. I highly recommend mechanical keyboards, they are very enjoyable to type on.
http://elitekeyboards.com/support.php#2