IBM Model M and Model F keyboards (and their heritage manifested in Unicomp re-makes) are known and loved even today for a number of specifically unique features:
- Buckling spring key mechanisms. These are known for their extreme tactile feedback - the buckling spring mechanism is unique in that it's not the operator's finger force that closes the contacts, but he rather only "loads" a spring until it buckles and releases the accumulated force onto the contact hammer, thus "mechanically isolating" the typist's fingertips from the actual mechanical force required to close the contact. A buckling spring keyboard requires progressive force to press until the buckling spring tilts from linearly compressed to "buckled" and closes the contact, and from then on practically none for the rest of the key travel. Buckling spring keyboards thus give extreme tactile (and, clearly audible) feedback to the typist on exactly when a keypress is registered. You could say a BS keyboard imitates the feel of an IBM Selectric mechanical typewriter.
- The IBM Model F and Model M keyboards are built like tanks, with the the "F" weighing well over 3 kg, the "M" still like 2-2.5 kg, which makes them basically indestructable (a Model F can seriously be used as a weapon...).
- The "audible feedback" the Model F and Model M keyboards produce might not be compatible with modern-day open-space office setups. For the typist themselves, it might be easy to adapt, but in today's office spaces the loud clickyness of these keyboards tends to simply annoy co-workers (also, be aware that everybody will notice when you're not typing ;) ). These keyboards were obviously designed in times when the rattling of numerous mechanical typewriters was generally accepted office background noise.
Model M keyboards can be considered the cost-reduced version of Model F: They use the same buckling spring mechanism, but replace the original capacitive contacts with a more mundane membrane. While this theoretically shouldn't really affect the keyboard "feel" (as described above, the typist's fingers don't really receive direct feedback from the actual contact because of the buckling spring), most people would say the "F" tactile feedback is even better (and noticably louder) than the "M"'s.
The buckling spring mechanism is unique for Model F and M IBM keyboards (and their descendants, the Unicomp keyboard) and original "F" and "M" keyboards are highly sought after, typically achieving high prices when showing up on eBay.
Unicomp keyboards use the exact same buckling spring mechanism as the IBM Model M, but are lighter (about half the weight of an original "F") and (your mileage may vary) of reputed varying build quality. Being available with modern USB connections and various layouts they are obviously much easier to adapt into a modern computer setup.