The IBM Model M is an old mechanical keyboard which started production in 1985. It uses buckling-spring switches, and can still be found in use today thanks to its original popularity and epic durability.
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Pros
Pro Tactile and "clicky" buckling-spring key switches
Buckling-spring switches are hardly used anymore, but they give some of the best tactile response when activated.
Pro Very sturdy
Although heavier than a modern keyboard, this keyboard is built like a tank. There is no flex, and the components were built to last.
Pro Solid construction
Quality construction, good materials.
Pro Clear separation of key groups
Pro Attitude
Clearly signals that I'm serious about what I do and how I do it.
Pro Removable / replaceable keycaps.
With a hook beneath the caps that you have to get to, you can detach those keycaps and replace them with others.
Cons
Con Noisy to type on
Floss mod is available for the Type M to make the keyboard a little more quiet. However, since it's still a mechanical keyboard, it will be noisy. This keyboard will not be appreciated in the office.
Con Very hard to acquire
Out of stock for decades.
Con Ugly as hell
Con Some driver issues with USB-only computers
Using a PS/2 to USB adapter can kill rollover.
Con Very large
Measuring nearly 20'' x 8.5'' x 2" ( 50 x 21.5 x 5 cm), this is a very big keyboard. It will be hard to fit into a minimalistic styled desk setup, and might even be too big for some smaller desks.
Con Lacks some modifier keys
The Model M lacks both of the meta/super/cmd/Windows (depending on the OS in question) keys and a menu key.