the original ibm model m keyboards dated pre-1990 are excellent in build quality, however, they aren't as responsive as new keyboards and they lack a start key.
i use one myself. there is a noticible pause when holding down the backspace key between the deletion of each letter. on occasion the keyboard also might stop responding for a second or two causing you to retype. i've tested this with both my model m's and with 3 other keyboards on two different machines.
overall the benefits outweigh the limitations but a brand new mechanical switch keyboard would be ideal.