The browns require a bit more force to press down (very relative mind you) and feel a bit crispier. They definitely sound "clickier" (higher pitch).
The switches in the HHKB are very easy to press down, but have a more solid feel, almost like a very smooth model M, it offers a nice resistance while at the same time being lighter. (both the M and the HHKB use springs in their switches, see
http://www.pfusystems.com/hhkeyboard/leaflet/keyspec.html for the HHKB Pro).
I do find myself being more accurate on the HHKB than on the brown cherries (that's been one of my minor complaints about them, although it's a lot better than in the beginning).
Still, I would say both of these are my favorites. The Filco (w/ brown cherries) at half the price of the HHKB is an amazing keyboard. Something cool about the HHKB (especially with blank keys) is the geekyness factor. Other boards just look like a regular keyboard (or an old vintage POS) to the untrained eye. This board makes a bit more of a statement if you enjoy that kind of thing (I do).
The design of the HHKB is very tight, has a small learning curve (switches and key combinations) - which I think, like learning vi/emacs/etc makes it a bit more personal of a board. There's a personal investment and effort involved. The layout is very intelligent and elegant, I don't miss having the other keys: function keys, numpad, arrows, etc. All of these are accessible through easy key combinations.
One big advantage is that you get an extremely good quality board that's extremely portable, unlike a model M or even a regular full sized board. The HHKB will easily fit in a laptop/messenger bag, is light (but still heavy enough to be stable on the desk) and has a detachable usb cable - just unplug the board and get a second cable for at work or retractable cable to take with you.