I have one.  It weighs about 3 pounds (1.3kg) with lens and film magazine
(which is significantly less than a Nikon F5).  I use it a lot while
traveling, and I like it.  The big advantage is the negative size.  Other
plusses are easy operation and a big, clear viewfinder.  It does have some
downsides, too.  The meter on the old 645 is pretty primative, and you need
to be aware of what it is seeing.  The 120 (or 220) film is not available
everywhere, and you need to haul a reasonable supply with you.  The camera
is not particularly waterproof, and a bad rainstorm can put it out of action
for a while.

Don't know exact weights, but my 645NII and three zooms are lighter than my
F100 and three zooms. It's a wonderful travel kit with a 45, 75, and 150.
Light and it covers a wide range.

It weights about 1.8 kg with film insert, batteries and a medium-sized
lens: not very heavy, in fact it's one of the lightest medium format
SLRs available when you consider the TTL AE prism, the powerdrive and
so.
I carry it with me in every travel I do. Have been in Iceland last
year, and enjoyed my 645 more than ever! :)
I normally travel only with the 35/3.5 (terrific lens!!) and 150/3.5;
if I need a longer tele, I carry the Rubinar 500/8 (mirror lens, very
compact and light). If I have enough space and don't have to walk a
lot, I carry my Arsat 30/3.5 fisheye.
I really love my 645... it always served me well, both for personal
use and for assignments.
Consider buying a couple of inserts: it's not funny to deplete the
ammunitions in mid-fun (changing rolls takes longer than with 35mm
gear). ;D

Fernando