I had been using an old Model M (with the blue IBM logo) for the last 7-8 years. Bought a Unicomp recently to get windows keys and USB.
It's awesome.
but two nitpicks:
The IBM Model M had two layers of key covers on each key. A blank peice that inserts into the keyboard base, and a cap with the letter inked on it that goes over. The Unicomp has the letters printed on the bottom peice, and no cap. I find that this effects the weight enough to be noticably different. I ended up swapping the peices over to the new Unicomp. Everything fit dead-on except for the Alt and Spacebar keys, and obviously the windows keys which are not present on the original IBM.
Second, the Model M had a drainage system (for when I spill my coffee :) that is absent on the Unicomp.